If someone is sending you messages you do not wish to receive, using the /ignore command, followed by the troublesome players name, will block incoming messages from that person.
If you are having trouble ignoring a user, it may be due to the fact that you have not correctly entered his name.
Type /ignore, then select the user and push the Ctrl and N keys at the same time.
Also, you can take advantage of the Do Not Disturb command to stop all incoming messages.
Type /DND, followed by the reason you do not wished to be disturbed.
This will send that message to people attempting to privately message you and you will no longer receive private messages.
Example: /DND I am busy trying to take over the world and can't talk right now.
Type /DND again to disable the Do Not Disturb function and receive private messages again.