It's not possible to block messages from people you've been in a recent transaction with. You wouldn't know they've blocked your bids until you tried to bid.
You can't put a blanket block on a seller but you can exclude them from your searches. Using Advanced Search (top right of any My eBay page, next to the blue search button), put in your keyword then scroll down to "Sellers" , tick "Only show items from", change 'include' to 'exclude' and put in their ID. Then click on the blue search tab at the bottom.