Since when is Shul NOT a place to mingle? (Just had to ask because it is a place for a community to meet)
I’m not even a card carrying Baal Teshuva… and I gotta agree. Rules are Rules. Put a sign in front of the Shul informing everyone that a proper dress code is required.
As someone in power, you have ever right to enforce the rules that you feel are important and restrict membership and/or entry to anyone who feels differently. Mail a general letter, and if that doesn’t work, then face the problem head on.
Either you’re going to have standards or you aren’t.