It will not help someone's website's Google ranking for them to post your URL (or link to it) unless, perhaps, you have a really high page rank and their site is about the same subject. It also should not hurt your site's PR.
I do not know how to stop them from doing that, other than to ask them.
One thing --- check your website's stats. Make sure that all the pages that receive traffic are either pages you put there or admin. There is a way to hack sites and place someone else's pages on your site, perhaps using issues with the "Front Page" program.