/tells are a big part of a game but gil spammers are out of control. Right now I have done like most people do and put all tells to a tab I NEVER look at which is a horrible way to handle the spammers. I would suggest either
1) separate tells out from people who you allow (using the friends list) and ones who are not. This would allow for us to continue to ignore the spammers but still get tells from friends
2) Make using /tell a level limit, just like trial accounts can't. Level 20 should suffice, it would at least discourage it a bit more because now they have to put forth effort to level only to be banned and do it all over again.
3) Do like most MMO's and create a filter where a person is auto banned for specific information which are in the tells, there are only so many ways they can put their w-e-b-s-i-t-e in.
Suggestions, but ultimately something has got to be done eventually.