Truth is, looking at the sign, I do not think there is even damage to H&M, rather, these bochurim probably did them a big favor.
The ad is still there. It still says H&M. You can still tell that they sell clothing. the difference is that the offensive part is covered.
people who may have turned away before from observing the ad now can look at it. (This is especially poignant for all the brilliant commenters who say, why do people have to look. The sign is there, should tpeople have to stumble on the streets of thier neighborhoods because someone would like to put untzinuyus things all over?)
So lets observe what the result is:
1) The ad is still there and still says H&M.
2) Had a tarp been spread, even H&M would be unvisible
3) More people can now actually look at the ad, because the non-tzniyus portions are covered, thus increasing viewership.
These bochurimactually did H&M a big favor. And if H&M had yashrus, they should actually pay them for expanding the potential market the ad can reach.