As with any messenger out there, it's possible to get unwanted messages sent to your account. For such an occasion, Surf allows you to block messages in any chat except bots.

Open the chat with the offending contact and click Block messages. The second option to block messages is to press the chat Settings button in the upper right corner of a chat screen and select Block messages. While messages are blocked, the text input field in the chat is inactive, and displays information on which of the participants blocked the other. 

You can unblock messages in the chat Settings menu you blocked them in.

Note, that coin transfers are not blocked by this action.