When should you block someone?  Well if you think they are unfair and keep harassing you, then yes.
But is it fair I ask, if you ask someone to not be so lude in their comments and they basically say F***-off.
The you comment one more time politely telling them that you were just being fair and thinking of the person they were being rude and lude to and their response is "Stop telling me what to do".  There after you have been blocked from any contact with them for all time?  Is this fair?  This actually happened to me over two non-inflammatory comments in defense of another deviantART member.
Is it fair that I will never get to know this person who may have great art in their gallery but I will never get to FAV it?  What if I were feeling generous and wanted to give this person a 3 month premium subscription?  In my opinion I think it would be more fair to put a limit on the block in terms of say months.  Possibly make permanent blocks part of the premium subscription but under the careful watch of an admin to ensure fair blocking.  You've heard my opinion what's yours?