A happy middle ground might be a designated billboard zone (e.g. Times Square), with billboards outside a certain area being outlawed, or restricted by size.

But the Devil is always in the details: how would businesses advertise their presence? A sign is an ad of sorts, and regulations would have to be drawn up for every type of sign that might exist: size, wording, color, etc, lest it be deemed too "advertise-y."

How about mom-and-pop businesses or sole proprietors? Would it be illegal to tack your business card to the local laundromat bulletin board? All these questions, and many more, would have to be answered in the quest to fully (or nearly fully) ban ads.