Hello Dear

You should just be able to click on the delete message and then a confirmation box asks if you are sure, and then click the delete in that. Just tried it on an old message and it worked ok.

By blocking the user you won't be contacted from him in the future.  But if you would like the message to disappear from your inbox you should actually delete it as Badger suggested.

Many thanks to both of you. Thought I had that but must have done it wrongly