In our attire section I have it saying "formal attire requested. Black tie optional." and then I have some attire recommendations like to bring a jacket/shawl etc.
Then I have it say
"To create the romantic and elegant atmosphere we desire; we ask that you do not wear jeans, shorts, sweats, t-shirts or other casual attire."
You would think that if you say "formal attire requested. Black tie optional" people would get the hint to not wear casual clothing. BUT I feel like some people just won't get it.