There's a tip here for finding out whether two accounts are using the same email address, even if they have their email hidden on the character's journal. If you draw up the same number on the potential-troll as you have on the journal of the definite-troll it is without question the same email address.
Mind, it's pretty easy to create new gmail or hotmail accounts, but I always pray that the trolls will give up because it's a bother.