Require a paid account to initiate a /tell.

Confused?

Free Account:
Can Recieve tells from others, enabling /tell with that individual
Cannot send initial tells.


End Result:
Bots will need to purchase a full account to initiate /tells.
Real players will still be able to /tell their friends, and freebies will be able to too... after the friend /tells them (also doubles as confirming that you have some means to initiate contact with that individual).
Because Bots will need to purchase full accounts, will force them to use /say - making finding them easier for players and staff alike. Let the bothunt begin.
Nobody's really hurt unless the newbie on the free account's friend is anti-social. In which case... why are they your friend again?

Question inb4:
"Why not make it so you can only /whisper friends?"

Answer inb4:
Because there's already bots who are adding people to friendlists, I dealt with one a couple weeks ago. Got sent 3 add requests out of nowhere, as did my former FC.