If you are an admin of that server, the command are like so.
/kick USERNAME
/ban USERNAME xxx < xxx is the ammount of days you wanna ban him or her for.
If your in a public server, sometimes you may see your allowed to vote to kick/ban. To do this, simply open up the connections list (press "n" until a box comes up with the names of people in the server. It will be in the bottom right.) then click either kick/ban by there name. This will cast a vote. If someone else starts to vote, press "1" to add your vote. You need at least half of the server to vote, for that vote to suceed.
I hope this helps.