If you mean "forever forever", like when they leave the page and come back, you will need to incorporate a server side technology.
If you have users who aren't registered, you could use PHP (or similar) to capture their IP and mark that IP with a flag. The flags initial state would be 'show' or similar. Once they have clicked dismiss, do a quick jQuery ajax request to your server marking the flag as 'hide'.
Obviously if they changed their IP you're info would be out of date.
You could also use cookies as a solution, but if they cleared their cookies it would be lost.