I would say to go for a plugin creation. We can have a plugin which will maintain the blocked user list for each user. Then the problem narrows down to just filtering the questions from the blocked list. I had made a similar plugin which filters questions from selected tags per user preference -- you may change this. This requires slight changes in the core -- the changed core file is also part of the below plugin.
As a second part, we need to block out messaging too. I don't think any Q2A plugin does this now.