Are you a board member or officer of your shul or is it just you being anal? Son, tznius isn’t for you to enforce, it’s for you to observe. We are now in the aseres yemei hateshuva. I’d be careful about judging other Jews if I were you. better worry about your own failings.
As a practical matter, there is no way to physically enforce a shul dress code that won’t land you in court or, worse, jail. Just look the other way.