TheMonkofDestiny: Look in the config (.cfg) file for
bind t "messagemode" and change t to a key that isn't already in use for another bind.
Should also be able to change it via console (press the ~ key and then type in the
bind t "messagemode" command, changing the t before hitting submit.
Alternatively, if you just don't want it to be bound at all, type
unbind t via console and it should unbind it.
Sadly the config.cfg file always gets overwritten by the game, whenever I start the game anew. Setting it to write protected doesn't change anything.
Typing unbind t via console works, but I have to do that every time I start the game. I would be nice to have way that works permanently.