There is a Google Opt-out Browser Add-on. It's something the end-user would need to install on their browser. If you're creating a site, you could add a link to it for visitors to add.
for this reason i ask if there is a ready solution. for example a js function to set the variable "useTracker" during the pagevisit - as a session variable. So the user can check or uncheck the tracking. for example a modal window that is showing up during the first visiting with a check / uncheck option and a few links to the terms etc.