Are you certain that anyone is doing it deliberately? After all, if someone uses their own name, it isn't all that unlikely to meet another user with the same or very similar name as them. Or if someone takes their username from popular culture, or it's a pun (like mine) it's not unlikely that someone else will want to use the same one.
Before I registered I checked to see if Norman Castle was unused. I didn't even think of checking NormanCastle or Norman-Castle or any other variations.
The solution is, if there is clear evidence of deliberate action you ban them as a troll. If there is no evidence, then it's likely just a co-incidence, and you instruct the newbie to pick a different name.