I don't think that could ever really happen. Leaving aside the fact that I don't think it's morally acceptable to interfere with other people's rights to advance your own (two wrongs, etc.), I don't think it could ever be effective. If you block someone from going about their normal life, their emotional response is to hate you. Even if they would have been sympathetic to your cause, they will now start rationalizing why you are "wrong". If they start calling on the government, it will be demands that they put a stop to your group. The best you can hope for is they will think "I agree with their goals, but these jerks go too far!"
This is what pisses me off every time HFCS comes up in a debate. You're not supposed to replace HFCS with sugar. You are supposed to replace it with fresher, less processed foods that don't need added sugars.
The problem I think was that 2 different anti-HFCS groups got some publicity at the same time. One was Dr what's-his-name who called HFCS "poison". But he really meant all sugars. It's just that HFCS was the main one found in everything at the time (because it's cheaper, easier to add since its liquid, and the corn supply is more stable than the sugar supply).
At the same time, the "Passover Coke" crowd was making noise about how much better CocaCola tasted with cane sugar, compared to HFCS. I agree that it does taste awesome, but that has nothing to do with health.
Unfortunately, these two movements collided in the public conscienceless and became "HFCS is really bad for you, and should be replaced with sugar". So now you have idiotic things like Raisin Bran that proudly says "No HFCS" on the box but is full of added sucrose. Raisins are supposed to be the sweetener in Raisin Bran, the only other ingredient should be bran.
"Ignorance is the soil in which belief in miracles grows." -- Robert G. Ingersoll