This is done on the Teamspeak 3 client when you're logged into your server.

To kick a user...

1. Right-click the user's name.
2. Select 'Kick Client from Server'.
3. Enter a message to display to the user.
4. Click 'OK'.

That's all there is to it!

To ban a user...

1. Right-click the user's name.
2. Select 'Ban Client'.
3. Select the duration of the ban (by default, this is a permanent ban).
4. Enter a reason for banning the user (Optional).
5. Click 'OK'.

That user is banned!