Next time you see it, perhaps try to take a screen shot of it (Ctrl PrntScr buttons at the same time on a Windows based system) and paste into a graphic editor saving it as JPG and then emailing it to email@example.com along with a description of what is happening. That may help MNHQ identify the advert in question so they can ask the ad-vendor to change how the advert behaves when hovered/clicked.
Today my MN browsing pleasure has been severely disrupted by the advert that I can see bouncing about like a loon to the right, underneath Discussions of the day and above Mumsnet Latest. It's (I think) the one that kept appearing on other sites which is trying to get you to enter a comp to win an iPhone and then hooks you into surveys.
It's now started itself up for about the fifteenth time when I accidentally dragged my cursor across it and the only way to lose it is to leave the web completely and restart. If you try to click the x key in it's top right, it just launches anyway. Same with right click mouse and Close.
Please dear kind HQ, give this lovely bottle of gin to Tech and ask him to make it go away.