Replacing tmux with exec

I'm using tmux from a while and I'm very happy with it. Since I need it most of the time, I made it my default shell (in guake, to be precise). This is nice and I'm perfectly fine with this. But from time to time, I need to exit tmux and having a clean shell, but since I'm defaulted with tmux, terminating the session will result in another tmux later. So, my question is: is it possible to replace the *tmux process* with a given shell? Something like exec which, of course, doesn't work, because it replacess the shell inside tmux and not tmux itself.