From: Southern California
I'm not familiar with Teamviewer at all, but if it uses ports other than 80 and/or 443 you can simply block that traffic (or not allow it in the first place and let the default deny rule drop it). If it uses port 80 you will need to use a protocol analyzer (e.g. Network Monitor or Wireshark) and observe the traffic for something unique to the communication that you can filter on (most often this is the user-agent request header). If it uses port 443 you will need to employ a third-party filter to perform forward SSL inspection (on ISA, TMG will include this by default).
- signature containing 'DynGate' (HTTP Header, User-Agent) - url containing 'dyngate' (just to make sure ...) - domain Teamviewer.com (to prevent acces to the Web access method at login.teamviewer.com)