It probably seems to stop because RMTs use the same account for upwards of 15 minutes to repeatedly spam people. If you're the first one hit and you blacklist them, you probably won't be hit again for a little while (and if you are in a dungeon or something you cannot receive /tells) unless there are multiple accounts spamming the same zone at that time. Square's not as aggressive about removing RMT as most people would like. The temporarily ban rather than delete because RMT frequently steal legitimate accounts and get up to shenanigans so SE of course wants to restore accounts to their owners rather than destroy them. Personally I'm not even sure how RMT exists in this game given that there's virtually no player economy and very limited uses for gil in excess of the amounts you receive from regular play.