How do I keep ads off my IMac

If you go to > Safari / Preferences / Security and make sure that Block pop-up windows is checked, it might help a little...? However the biggest problem is that many sites find ways to make big money passing those pop-up ad's thru, regardless of how good the pop-up blocker is. Personally if I run into a site that is pushing a lot of flaky pop-ups, I stop going to that site.

Use Firefox. I almost never see any ads at all.

Firefox will let you do things no other browser will.