Avraham Avinu looked at Sarah once, too, to see that she was beautiful. Perhaps they looked once and then to prevent themselves and others who may not have seen it from looking excessively,they felt that this drastic measure was justified.
As mentioned above, though, defacing H&M’s property is not correct, legal, or justified. The bochurim probably have not thought out their other means of non-violent opposition, such as contacting the company and asking them to change the ad themselves, or tacking a raincoat on the billboard, which could be removed without actually destroying the property.
BUT, this is not the first such billboard in that location that is not tzniusdik and it’s not the first one to be dealt with in this fashion. So why does the advertiser keep posting there? Who is the great genius in their market research division that tells them 16th Ave and 43rd (?44th?) street in ultra-religious Boro Park, is a great place to advertise trampwear? Ah, there is a public school across the street. Perhaps the advertiser’s money would be better spent sending flyers directly into the school or subsidizing some aspect of the curriculum, which could not be affected by these young, zealous guardians of our purity.
Let’s hope someone from H&M reads YWN! (Moderators, maybe if we help them clean (or dress) up their act, they’d even advertise here!)