Just for the purposes of clarity, PIA doesn't spy on anyone or invade anyone's privacy etc. The ads you see are served by Google. - Google serves you interest based ads. If you want to opt out of Google's interest-based advertising (across ALL websites that serve Google ads), then here's the link - http://www.google.com/policies/privacy/ads/#toc-optout (and the page also explains how Google determine what ads you see).
Sorry if I wasn't clear. I also know it is Google - that is what is scary. They know what we are doing. Still don't get the PhD ad!!
I didn't know you could block it so thanks for the tip.