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.
Hey folks we've come back to update this idea status as 'case closed'.
We are currently in the process of deprecating the Inbox/Messages feature within the Spotify app.
Extensive data analysis has shown that this feature has very low engagement. The huge disparity between the use of the feature and the manpower required to maintain it doesn’t merit keeping it running, so we've taken the difficult decision to remove it.
It’s still very easy to share and chat about music from Spotify. You can check out this support article to discover other ways for getting social. Thanks for your continued feedback everyone.