I'm not aware of Mp3tag internals, but from where I stand, simple check whether the list of files is in edit mode to not to override default clipboard behavior should be sufficient. Even Ctrl-V is the problem, we still have plenty of other unused keys in the vincinity of Ctrl-V: Ctrl-Q, Ctrl-W, Ctrl-E, Ctrl-F, Ctrl-B. Any of these could "paste" the folder into Mp3tag without messing with default clipboard operations.