Look for the Competitive Ad Filter in your Google AdSense manager. (https://www.google.com/adsense/filter-online -- you'll have to be logged in to get there)
Add plus500.com to the list of blocked domains. It doesn't really have to be a competitor. I have a huge list, from annoying to non-matching, to block low paying ads.
Note that this won't work if those ads are being served through another party like an ad network. In that case, you would have to block the ad network too.
Right click on the ad (don't click on it for real) and choose Copy URL. Then go paste that URL in a text editor. Somewhere in the middle you will find a domain name. Block that domain name. With Firebug, put your mouse over the ad and right click to choose Inspect Element and then do the same as above.
You can't block Flash ads this way since the source URL is not where the ad actually comes from. For that you'll have to peek into the Flash code. Here is a tip on finding out where flash AdSense ads come from.