Even with modest storage space, a forum can old millions of users and posts, and the limit for integers is beyond 2 billions.
vBulletin forum has 318007 users when I write this message (more than 7000 of them being active), and
phpBB forum is well past a million users already.
A botnet can continue attacking for months or years if nobody changes the targets. The only reason for them to stop is if it becomes more "productive" to attack elsewhere. By reducing their spam next to nil, we make our forum less interesting for them. Unfortunately we have no better solution than taking the spam in the face, wash it away, and wait until they see it is of no use. By reporting the spam that you find, you contribute to cleaning up the forum very fast, as our system will prioritize reported posts containing the word "spam" (and for this reason, make sure to spell it correctly, the system may fail to recognize the word if it has a typo).
Restless guardian of this forum.
Despair spammer ye who enter here.