Well, while that is immature on their part, that may be because you have pissed off a lot of, albeit immature, people. Find out why they are pissed at you (if you don't know already) and then change your behavior for the better.
For example, if you have a huge reputation for mistreating and disrespecting the staff and members on your forum, then you may want to publicly apologize (and without b.s. excuses, just own up to your behavior) to anyone who still stuck around, and learn to treat your members with respect and as they deserve to be treated. This way, when these users come around your forum with these names, and they see that you are not the person that someone else said you were, then the original person will look like an ass, lose credibility, and who knows maybe the attacks will stop.
However, whether that is or isn't the case, you still shouldn't be disrespected like that, and that is not to say that you wont run into the occasional a-hole who just wants to start trouble. What I would suggest, is that, first, turn on admin validation. This way you can preview all accounts, before you validate them. People can very easily register email accounts that are defaming you, provided that they don't contain obscenities. At least if you have on admin validation, you'd HAVE to manually approve or deny each account.
Second, if possible, add your username to the word filters, this way you'd at least be able to prevent anyone from using it (much like obscenities) in their usernames. This shouldn't affect the quote feature on your forum, but just in case, you should make a test board with the same software, and then test it out. You should also add the names you've seen so far, to the banned list.
Third, temporarily disable their accounts, so you could then wildcard ban their ip addresses from the forum, you should be able to keep banning the ips as they keep making new accounts. However, if you have a limited amount of space then this could be a problem. You should also make a note in a private area, viewable only to you (such as a textfile on your computer) of what ip addresses were associated with which offensive accounts. This way you would know if it is one person, or many others. And if it is one person doing it, then you might be able to file a complaint with their isp.
Finally, you could then delete their accounts. This way you will have all of the names banned from the forum and no one can re-register them once they've been deleted.
I also agree with the poster who said that making a global topic is a bad idea. That just encourages their behavior.