First off, are you typing in the server console or the client console? Either way the command is kick or clientkick, not boot. If you are using the server console, you can type status to get a list of players and their client numbers. Then type clientkick # and they're gone. If you're using the game console, you either need to set to your rcon password using /set rconpassword yourpass to whatever you have in the server.cfg (or whatever config file you use) or include it in the command like so:
/rcon rconpass kick playername
Just substitute your specific rcon password as needed. I find it ten times easier to kick from the server console using clientkick. But, that's just my opinion.