@harrymccracken AFAIK that's the major weakness of a federated system like Mastodon. The same username may be used on each individual server, since there is no central authority to deconflict them.
You might try contacting those users directly--or the admins of those individual servers to ask that they be removed, but that is a crap shoot, especially if they're bad actors intentionally copying your presence here.
Good luck and keep us posted.