Let's give this a go.
We all buy petrol. Petrol companies are in competition, yet the prices are all similar. Aboout 60-70% is tax, but there's no price variation between companies on the remainder. Sooooo - let's all choose a company. Let's say BP. And let's boycott it. With no sales, their prices will drop dramtically in a week or two. Now, let's not get greedy, that's just the start. We keep boycotting BP and then do the same for Texaco. When they drop their price, then we start to buy from BP and ONLY BP. Texaco will stay low, and all others will now effectively be boycotted. Within 48 hours all prices will drop.
The downside is that I know no-one would wait and would hit BP as soon as they dropped prices. But it's an idea...