In Empathy IRC, how do I turn off the "joined" and "disconnected" messages?
At the moment, there is no easy way to do this.
There is currently an open bug report regarding this issue: https://bugzilla.gnome.org/show_bug.cgi?id=599453
In the meantime, I'd recommend switching to xchat or irssi, which are both great chat programs (xchat is GUI, and irssi is CLI), and are a lot less buggy than empathy. (unless you feel like testing/debugging/extending empathy)