Right-click their name and choose Kick. Type a reason; they see it. They can come back at once, so it is a warning.
Ban someone
Right-click their name and choose Ban. Mumble bans the connection address, and if the person is registered their certificate too. Add a reason and, if you like, a duration.
To undo a ban open Server, Banlist, select the entry and remove it.
Mute or deafen
Right-click a name and choose Mute or Deafen to stop that person speaking or hearing. Use Priority Speaker for someone who should be heard over others, such as a game leader.
Move people
Admins can drag a name to another channel. The Move permission controls who may do this.
Give moderators the right permissions
Create a group mod on the root channel and add your helpers (registered users only). In the root channel's ACL, add a rule for mod that allows Kick, Mute/Deafen and Move, and nothing else. Keep Ban and Write for admins. See How to become admin on your Mumble server.
Good to know
A ban by address can catch other people on the same network. Use a short duration when in doubt.
You see who is connected in the channel tree, and you can read the server's log in Console in the panel.