I suggest to block tells from trial accounts unless they in friend list.
I.e.
1. If you have regular account you tells will not be blocked unless person specifically squelched you. ( no changes here ).
2. If you have trial account your tell will not be accepted by any players by default - unless you added into friend list( or if friend list is not a good place then create new list, but that is more work).
That way ppl who want tried game and still will be able to chat in guild/channel/group or general channel, and if they real ppl they will be added into friend list on request, but direct tells will be disabled by default. As soon as person getting real subscription this restriction will be GONE
I think this approach will finally solve "TELL" problem.
I am not sure how easy or hard this is to implement. But I assume that it is possible unless tells go direct pc->pc without server, in that case for each tell pc will have to ask server "accept tell ?" question - which is more work and extra time to check.