Unfortunately it's something the host decided to do. Basically Forumotion has decided that if Your forum is considered inactive(no posts on it for a certain amount of time) that it sends a popup once a day to every person who visits any forums associated with Forumotion. People have complained about it to them, But they just don't seem to care.
I myself have a site that is on Forumotion, I am hoping to get away from them by the end of the year.