Short-term, certainly. Long-term - negative ROI's would drive away the real customers, who are paying for those ads.

Or at least that's the "Optimism set to 11" plan.

Evil plan: Create a network distributed system (kind of like Seti@Home) specifically for doing this. If there was enough participation to make a difference the advertisers would have to invest in countermeasures which could lead to an arms-race that ultimately erodes all profit from it.