If you're hosting the server, then the console command is KICK "PLAYERNAME", (where PLAYERNAME is the actual name of the guy you're kicking.
If you're not the host, and the admin isn't there to kick the offending player, then download my Voting Tools from the mod section of this website. Here is the quick link,
http://www.alliedassault.com/downloads/mods/Elvite's%20Voting%20Tools.zip . There is a README file included that explains how to install and use it.
Even if you usually host games, my Voting Tools are good to have as they automatically unlock the command console without having to know about command line variables. If you don't know what I'm talking about, then you need to download.
Good Luck