We do not have the right to vandalize. This is not a Jewish State or even a Jewish City under Jewish rule. So OUR absolute standards of tznius (which vary in many respects among frum Jews),
have little shaychus to the goyim. THEIR standard of tznius, as long as it does not violate common accepted decency (and the wearing of pants and even sleeveless shirts by women, is accepted as modest attire in this country)should not be treated in such a holier than thou manner. Suppose they had a giant ad for a ham sandwich or Red Lobster? Would you think they had the right to black it out? What we hold for ourselves we have no right to expect people who are NOT unzerer to likewise hold for themselves.
It would be wonderful if everyone dressed appropriately to our way of thinking, but that is NOT our call to make. Not in this country. Supposing a Muslim had done this and not those guys.