The key thing to remember is that the forum software treats each alt that you have as its own account.
So someone posts something trollish or spammy on one alt. The forum moderators look at that character’s posting history, did they do anything like that before? No? Okay, delete the thread as a warning but apply no penalty.
The person switches to another alt, posts, deletes the character afterward. The forum moderators look at it and go, did this character do anything like this before? No? Okay, delete the thread as a warning but apply no penalty.
It only gets to the point where people who are dedicated to rolling up new alts and being disruptive and pervasive before they really start digging in and doing research, because that takes time. They don’t have good tools to do it, because of the way the WoW forums function, where none of the other Blizzard forums work the same way. They play whack-a-mole with individual posts instead of being able to easily see a pattern of behavior. For that reason, trolls can run rampant for way longer than they should be allowed to before anything significant actually happens to them.