If you look up towards the top you can click on Tools, there are scripts there and I believe there is a blocking one.
Reebs is correct, the MTurk hits database script allows you to block requesters HITS. I've done this with crowdflower and crowdsource. The only thing it does is makes the description a smaller section in the search window. I wish it would completely remove it from search but it doesn't. Hope this helps!