Administrators can ban players from their servers if they see fit. It's an extreme measure but it's their server so why put up with disruptive, abusive or cheating players? Bans can be applied to individually named players and/or their IP address and/or whole ranges of IP addresses. Bans can have whatever duration the administrator deems appropriate, ranging from minutes, through days, to permanent.
Don't worry though, play nice and you won't be banned!