In the new Messages tab (in v0.9.8+ which has rolled out to some users, if you don't have it yet it should make it out to you sometime in the future) it is possible to block people from sending you messages. When you block someone you get a confirmation window warning blocking can not be undone:
I do not find this a satisfactory solution, just like with Facebook I would like the ability to block and unblock people as I like. Just like relationships between people, blocking should not be a permanent undoable thing.
Thanks for your feedback around the Messaging feature. Right now this isn't our pipeline but we'd like you to continue giving your comments/ kudos here. If this changes or we have any more news we'll post again here.