I am not an expert on this but a quick look shows this forum is run by SMF (Simple Machines Forum) software. Which is both free and open source. Nowhere on their website, help forum or the online manual can I find anything that indicates there is a limit on the number of users. And from a logical viewpoint that makes no sense with forum software, users do not take up room like at a hotel, just an entry in a database.
So before we get users closing accounts perhaps it would be a good idea to find out if that is actually necessary. I do not think it is, though I have occasionally been wrong:).
I do see they are running version 1.1 and version 2.0 is available with 2.1 available as a beta.