RewriteEngine On
RewriteCond %{HTTP_REFERER} ^http://cgi.ebay.com/
RewriteRule /* http://www.yoursite.com/restricted_url.html [R,L]
RewriteCond %{HTTP_REFERER} ^http://cgi.ebay.co.uk/
RewriteRule /* http://www.yoursite.com/restricted_url.html [R,L]

That's a minor tweak to a copy paste from the wiki. If that doesn't work, I'm giving up for now (sorry).

You'd need to put up a restricted_url.html page or live with 404's, I suppose. A page would deliver an appropriate message if you wanted.

A referer blocker is something that removes or modifies referers being sent by your browser. For example Firefox has a plugin and some firewalls can do it. A standard setting might be to always send a referer of the homepage of the site being requested from (i.e. your site in this case). If you (or someone else) were using one, it could prevent what you're trying to accomplish.

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