here are the steps to block a bidder on eBay:

1. Go to the Block Bidders Page.
2. Enter your merchant account and then select “Block bidders or buyers from your listings” from the menu. If you don’t remember the link, you can search “Block buyer” under “Help & Contact” at the top of any eBay page. Your destination page will also be reached by clicking the first link that pops up.
3. Enter the Buyer’s Username or email address.
4. Click Submit.

That's it! You have successfully blocked a bidder on eBay.