There are two ads that I have blocked a long while back, and they are now showing up again. I checked, and the URLs are still showing as blocked. Has anyone else had this problem? If the ad block doesn't work, there is no sense in using it.
Depends on how you blocked them. And depends on how they're served.
Sometimes they use a redirect so you can't even see the correct URL. They're not supposed to, but it happens. Also I think some of the third party networks might do that (with permission), but I'm not positive. All I know is that there have been times when I tried to fish out the advertiser URL and couldn't.
If it's an image or rich media ad, you could block it, but advertisers can get around that by uploading a bunch of identical ads. Each ad is assigned an Ad ID# when it's uploaded, and there's no rule against using the same image for multiple campaigns and ad groups. If you block one in the Ad Review Center, I believe you're probably only blocking that particular Ad ID; there still may be another dozen or more (with different Ad IDs) that will still run. So the thing to do there is upload the image and tell it to find all the similar images, and block 'em all.
The same is true of text ads (multiple identical ads) but they're not so easy to block them all - you'd have to block by advertiser URL there, as there's no tool to find them all at once.