Don't. I use a password vault app to transfer in user IDs and passwords via the clipboard. Without clipboard paste, I'd have to display the password and enter it manually, which is less secure than pasting.
It would be rude to say the least to disable this function. I realize that people can do unsafe things using copy/paste - like mistyping a password and pasting the mistyped password in twice on a password check, but there are limits to how much obligation there is to protect them from their own stupidity.
Note that the copy function cannot copy a password OUT of a secret input control, only into one. That's true of all known browsers and if you find one that behaves otherwise, consider it to be unsafe.
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