I do know how to solve your problem, and I gave you the exmple with cookies. A session variable could be used too I guess, but that depends on how often you want the ad page to show up.
There are lots of scripts both on this forum and the web for setting and getting cookies in JavaScript if you don't have server scripts, search and you'll find them. They all boil down to giving document.cookie a string with a key/value pair and appending the expires key with a time value you see fit.
I simply give my point of view which might be a hint of what your visitors might think when seeing the ad page.
Btw, that script tag isn't on the site, it might be added by an anti-virus program or similar to prevent popups or other annoying things, or in the worst case it's some malware doing something nasty when you're surfing. I don't know what it does or where it comes from, but having a site linking to a local serverscript seems strange.