To block a member, simply go to the Messages section of your profile and click on a message sent from the member you wish to block. Click the Block Sender button.
You can also block a member by clicking on their screen name from anywhere on TripAdvisor. That will take you to their profile page, where you can click Block Member.
You can view a list of members you've blocked and unblocked by going to the Messages section of your Profile, clicking Settings and selecting Blocked Senders.
To unblock a member, simply go to the Messages section of your Profile, click Settings and select Blocked Senders. Click unblock next to that member's screen name. You can also unblock a member by going to their user profile and clicking Unblock Sender.