This may have been asked before, but:

Is there a way for me to block a particular thread or topic so that it doesn't come up in my "unread posts" list and I don't have to be irritated by it?

There are one or two topics on the forum at the moment that I don't really want to see or hear anything more about.

I know I could always block the individual posters, but however irritating they may have become on the topics in question, they may still have something useful and interesting to say on other topics.  So I don't want to go down that road.

Any helpful suggestions?

You could just not open the threads you don't want to see, unless you are suffering from Mary Whitehouse disease 

Read the ones you want to, then click "mark as read" for the others. That works well enough for me.