You can block any user by simply clicking their name in chat and selecting ‘block’. Blocking a user will filter anything they’ve typed from your chat window, past or present. You also can set up keyword filters as well. To do so: 

Please be aware that if you ever have to reset your game cache (a common troubleshooting recommendation), your chat settings will be erased.

As a note, it is common practice to block people who are chatting OOC during RP scenes. This keeps the chat from scrolling so that you have an easier time following the roleplay. However, the OOC chatters are often innocent and don't realize they are interfering with RP, so it's also common to unblock them once the roleplay is finished.