I've heard of a few people struggling to disable the Klipper in Plasma 5 (see this thread, for example.)
Some have proposed to completely remove the folder containing the executable, which is not a great solution because it makes the clipboard difficult to reactivate if ever needed, or simply makes it unavailable to all users using the same DE on a single system rather than making it a per-user preference setting.
Here I will explain how to deactivate Klipper completely, rather than simply making it invisible.

1. Erase your Klipper history by clicking on the Klipper icon in the system tray, and then selecting the broom icon. You will be prompted on whether you really want to erase the clipboard history. Confirm that this is your intention. If you would like to keep only the very last item in your clipboard, simply delete all the other entries using the red trashcan icon that appears when hovering over any of the entries in the list.
2. Right-click on the Klipper icon in the system tray and hit [C], or simply click on the "Configure Clipboard" option. Stay in the "General" tab and configure the options so that they are similar to that shown in the image below:
   
3. The only thing left to do is to remove the clipboard from the system tray icon. If you've followed all the steps correctly, it shouldn't be tracking your clipboard history anymore. Simply right-click on a blank area in the task manager at the bottom of the screen (ex-Windows users might refer to this as the "taskbar") and then hit [H], or simply click on the "Edit Panel" option. Hover over the Klipper icon in the system tray and click on the red trashcan icon that appears. You're done.

Steps to reactivate Klipper:

1. Right-click on a blank area in the task manager and hit [A], or simply select the "Add Widgets..." option.
2. Using the search bar in the widgets panel, find the "clipboard" widget (typing "Klipper" will not work, as the name is deprecated since I-don't-know-what version of Plasma.) Drag the clipboard widget to where you want it in your task manager.
3. Press [Esc] a few times to get out of the panel editor menus. If that doesn't work, you should be able to locate the X at the top-left of the widgets panel.
4. Right-click on the clipboard icon you just added to the task manager and hit [C], or simply select the "Configure Clipboard" option.
5. You should be in the "General" tab by default. Locate the "Defaults" button at the bottom of the window (next to the "Help" button in blue), and click on it. Click on "Apply" and then "OK"

NOTE: If you've been fancy and decided to edit the themes within your Plasma DE, then your icons and colours may be different to those described above. On another page, make sure you remove the clipboard from the task manager, in addition to simply applying the settings mentioned in Step 2. Otherwise, your clipboard might start behaving quite oddly and automatically copy things that you did not explicitly ask for. This is a bug, and disabling the clipboard icon is the only way out of it, as far as I know. I did not file the bug because it is insignificant. Anyone setting the keyboard history size to a single item obviously has the intention of making it behave like an ordinary clipboard such as that on many other Unix-like systems.

Disclaimer: The clipboard history feature included in the Plasmashell may be useful to many users. This tutorial explains how to disable it, while not at all claiming that it is universally useless. There is no need to advocate the use of the Plasma clipboard here, as I did not criticise it and respect the developers for their efforts in bringing this feature to the people who might find it useful. If I thought otherwise, I would have instructed the readers to just delete the executable's directory, which I explicitly disrecommended in the beginning of this tutorial.

Cheers.

I've found a much, much simpler solution than that above, while not even affecting the clipboard preferences setting at all. Read on for the steps:

1. Right-click on a blank area in the task manager and hit [E], or simply select the "Edit Panel..." option.
2. Hover over the system tray and select the "Configure..." option that appears.
3. In the system tray settings windows that shows up, you will be directed to the "General" tab by default. Locate the "Entries" tab (the one with the bell), and click on it.
4. Scroll down and find the "Clipboard" entry, under the "System Services" header. Disable the clipboard using the dropdown menu located alongside the clipboard entry icon and title, as shown in the image below.
   
5. Click on the "Apply" button and then "OK". You're done.
6. (Optional) Re-enabling the clipboard feature is as easy as selecting the "Show When Relevant" option in the dropdown in Step 4.

Cheers.

Thank you for this.

I'm truly blown away that security was never considered for the clipboard. They referred to this as a possible Privacy issue in the thread you linked, but I disagree and consider it a Security issue. Most *good* password managers offer to clear your clipboard after a configurable timeout precisely because of the security implications of having sensitive just sitting in your clipboard history.

I can't imagine that a simple timeout setting couldn't be implemented to have entries just expire, and even clear the entire clipboard when the system is locked. I realize you can manually clear the clipboard, but I personally don't always remember to do this after copying sensitive information, and I would expect a modern DE to take care of this for me.

Disabling it seems to be the only workable solution, and this helped because it isn't very obvious how to do this otherwise. There really should be an option to disable it directly from the configuration screen, or at minimum be able to set the number of entries to 0 to effectively achieve the same result.

Thanks again.
Cheers!

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Reminds me a little of the movie "Sleeping with the Enemy" (Joseph Ruben, 1991). In case you haven't noticed, the content in Klipper can now be removed via mouse / shortcut.

This serial used to be good, but it is now completely absurd.Saheba Brar knows that Angad left her because of the Shah family, but she is still moving close to the Shah, Dimple arrived because of Saheba, and she wants to attend a dancing academy despite the fact that Angad is a business titan and knows how his bhabhi and Saheba are doing. Director and producers are giving a false message to the public; cease this series right away or make Angad Saheba's life right to avoid legal trouble. 5:21 AM
Teri Meri Doriyaan