While I understand the sentiment, like all attempts at outsmarting stupid people, it is misplaced. If they're not copying your fake private key they're just copying someone else's.

In that case, just retract _any_ part of the key, such that it won’t work if you copy paste it, but still no used secret is revealed.

If a user wants to rebuild the key in order to use it for something, then they must have a very good reason to do so, and it is probably better to just let them do that.