I need to erase rectangular areas in screenshots an make these areas transparent. The screenshots are in PNG format in RGBA color space. So they have an alpha channel.
I prefer to avoid juggling layers. I just want to use an eraser (or something else) to make some parts transparent with as few clicks as possible.
I was under the impression that Kolourpaint can do this in a very simple way but I did not manage to accomplish this.
How can I erase to transparency in Kolourpaint or any other graphics program?
The shortest click path wins

GIMP can do it:
Use the rectangle select tool to select an area, and then click Edit--> Clear
This works for PNG images with alpha channel.