You can never prohibit this as you can not check if the TIE x TIE was arranged. Unless you take the job of checking everybodies messages of all TIE x TIE games of course;)
TIE x TIE arranged should be prohibited and liable to punishment. Combine TIE x TIE, affect the other competitors and can not be allowed.I and many other players would like an official position on combination of TIE x TIE.
P.S. What are BBs waiting? Why don't they make me an GM?
not even then... as (almost) nobody will be stupid enough to use BB-mail for illegal arrangements.