There isn't currently a way to "block" other users.
If you unfollow someone, they won't be able to direct message you, but they will still be able to answer your questions or comment on your answers, because the question pages are all public.
However, if you are experiencing issues with unwanted behaviour, the best thing to do is report it via https://blurtit.uservoice.com and we will aim to resolve any dispute quickly and amicably where possible.
