If you'd like to pause testing for a particular site you can do so by:
Going to your Settings page.
Clicking on the site you wish to pause testing for.
Scrolling down the page and un-checking all the test times.
This is helpful if you have an old dev version of a site that has now gone into production. You can pause the testing so it doesn't use up any of your checks but you'll still be able to see the old site data if you need to.