Why do you care really? Spammers could just as easily link directly to your homepage. Would you take your site down because of that?

The URL you mentioned is just a non-menu-linked URL for a category in K2. You can get similar URLs from any other Joomla component. The URL may be valid if it does not belong to a K2 category that is linked to a menu item. If you redirect it, you may be redirecting valid content.

If you're being sent traffic from specific sites, the sensible thing would be to simply block these sites as referrers. In other words, any link on these sites that points back to you can be redirected to somewhere else.

But again, is it worth it? I would say "don't bother".

I just noticed you use JSitemap. If it crawls K2 entirely then maps content to XML sitemap files, that could be the culprit if you don't link to these categories you mentioned anywhere else in your content.

It's simple really. Download all XML files that JSitemap generates and search for the string "component/k2/itemlist/category". If you find such URLs that point to content you don't want referenced in these XML files, look for any options in JSitemap to limit its crawler on K2 content.

What's the workaround? (may help others in case they ever face the same issue as you)

Thank you.