No one has the right to deface another person’s property. You do however have the right NOT to look and to boycott the store or the owner of the property in question. You also have the right to call either one with complaints. As far as covering it up, that is also an option, probably not legal but less problematic. Anyone has a right to advertise anywhere. AND for your information there are a lot of people even in Boro Park who do shop at H&M. So whether the ad was appropriate or not, the store was IN their targeted market and they do have a right to advertise there. Whether or not the owner of the spot should have reviewed the ad or should have the right of approval is another story altogether and maybe the community should take that up with them.
The Bochurim who took matters into their own hands should be punished and explained what they did wrong. The ad costs the advertiser a lot of money and THEY should be compensated.