Stopping bots is hard, but I'd think it would be easy to stop RMT spam though. Limit the size of tells from people not on your friend list. Limit the number of tells you can send per second. Limit the number of people you can send tells to in a day based on level. Allow people to reduce/remove tells from non-friends/fc. Put a play-time requirement on the privilege of sending more than one tell. And so on, and so on, and so on. Sure, they can get around the limitations, but if you put limits on them that take time to circumvent, it reduces the amount of spam they can generate per account.

Also, make blacklisting be per account, not per character. Seriously, why hasn't this been done?