You can ban players on your server. Be sure to use this ability with care, so you would be fair and consistent to your players, otherwise some people could be alienated. I'm fairly sure there's a command to ban players, there's a wiki page detailing all the commands and variables, you can probably find it there. You can also type help
in the in-game console (open (and close) with the ` (backtick) or ~ (tilde) key on your keyboard) to see all the available commands and variables. (You can scroll up and down the in-game console with the PageUp and PageDown keys on your keyboard.)
There's also a way to directly message a single player: First, type the command nodes
in the in-game console. This will print a list of all connected players together with their assigned node numbers (the first number, on the left, in each printed line). Then, type the following command, replacing the relevant parts:
sayto NODE_NUMBER_WITHOUT_LEADING_ZEROS YOUR_MESSAGE
For example:
sayto 9 Why are you behaving like this?
Or:
sayto 13 If you won't stop harassing other players, I'll ban you