There's no way to permanently hide a seller's listings. The best you can do is, each and every time you do a search, use the left filters to exclude that seller's listings. If it's a regular search that you run, you can save the search with the seller excluded and access it from your saved searches each time.
To exclude the seller, click on Seller under Show Only at the bottom of the left filters. If Seller isn't showing then click on More Filters then click on Seller in the pop-up that appears. You then have the option to include/exclude sellers by adding their id and clicking Apply.
You can also report any/all of the duplicate listings using the Report button on each listing page but it's not really worth wasting your time as ebay never take any action.
