You cannot simply block a seller the way that a seller can block a buyer, but you can exclude the seller's items from appearing in a specific search if you know the seller's username.
From your search results, go to the Advanced Search page to the seller section, select "only show items from" and then check "exclude". Enter up to ten seller usernames separated by commas and hit "search". Then bookmark or save the search results.
If you use the bookmarked or saved search results as the basis for further searches, you can continue to exclude those sellers' items from your results.
