Better idea... implement a cvar that can disable it or enable it anytime you want, removing it'll annoy some people. And what's this thread doing in Bug Report Forum?
There is no way to disable "friend is online" notifications.
Originally Posted by Enonatan
However, pop up menus can be disabled by going to settings -> account settings -> change site preferences.
There you can uncheck all the entries that start with "show me when a friend.."
You could also try disabling any anti-virus software that you have loaded when you are playing Quake, I have to dot hat with Trend :\
Doesn't have an impact on other games I play but does for Quake Live, gets annoying when I forget to disable it though!