You can actually disable RealID ingame by blocking a port with Windows Firewall. I enable the rule every time I bot, so it shows I'm offline to all my friends. Pretty sure guildies can still see you though.
To do this just make a new rule in your firewall blocking outbound access to port 1119. It's extremely easy to toggle on/off with Windows Firewall.
P.S. - Make sure to enable the rule after you log into your account, or else you won't be able to connect to the server.