I accidentally did a wheel button click and a postit looking thing appeared. How can I turn it off or get rid of it from my desktop?

Click on the sticky note and press on the 'trashcan' icon to delete it.
https://i.imgur.com/mLJLM2g.png

You can remove it by click and hold with the left mouse button, a little remove icon will appear next to it. To completely remove this feature of the middle mouse button creating notes you can right click on the desktop > "Configure Desktop and Wallpaper..." > Mouse Actions > Click on the minus icon in where the Middle Button appears in the list