Not really an answer - but the user id is the unique part, the name is just the human (and possibly SEO) readable bit, so there's no need in the system for the name to be unique.
Obviously there are any number of "John Smith"'s or "Fred Bloggs"'s who are using their own name - which is as it should be. However, I've never come across a user who's tried to impersonate a high rep user by using their exact name (I've seen a couple of "jon_skeet"'s and even a "john skeet") but even if I did their reputation would give the game away, so I'd normally be 99% sure it's the right person.
Have you got any examples of where this has been done?
With regard to your system - if you had a "score" generated by the system and that was always displayed, it would be even rarer that the user you were trying to impersonate would have exactly the same score as you. On SO there are 4 values you'd have to match (pure rep, and nos of gold, silver and bronze badges).