I've not ever seen pop-ups on this site in the 2 years I've been visiting.
Could it be from another site you might have up in the background?
I don't know if it's MR or not. The admins need the URL and a screenshot. If it is from MR, the admins can contact the ad service and have it removed.
From the FAQ: What do I do about annoying ads.
^^^ Can you post the link it refers to?
^^^ Can you post the link it refers to?