This setup below should work:

The only limitation is that people will qualify for the activity url when they view that primary URL (http://www.mysite.com) regardless of whether or not the query "ignorethispage" = true. If that is a problem, you can account for this by simply adding the negative condition (ignorethispage=true) to your audience and you will be covered.
Hopefully that helps
- Jon Tehero, Sr. Product Mgr, Adobe Target