I had the same problem with the middle button pasting even though I turned it off in the dconf editor. It turned out that the problem was only persisting in Mozilla Firefox and Thunderbird. They seem to have this functionality enabled by default as well.

This is how you can turn it off:

Firefox: Type about:config into the URL/search bar, click 'I accept the risk', search for middlemouse.paste and set it to false

Thunderbird: Go to Preferences > Advanced > General > Config Editor, search for middlemouse.paste and set it to false

I had the same problem with the middle button pasting even though I turned it off in the dconf editor.

It turned out that the problem was only persisting in Mozilla Firefox and Thunderbird. They seem to have this functionality enabled by default as well.

This is how you can turn it off:

Firefox: Type about:config into the URL/search bar, click 'I accept the risk', search for middlemouse.paste and set it to false

Thunderbird: Go to Preferences > Advanced > General > Config Editor, search for middlemouse.paste and set it to false