A "Lesbians not welcome here" sign would not be illegal. Then if they patronize, give them lousy service. A baker, for instance could reluctantly bake a wedding cake for lesbians and accidentally spill a cup of cayenne pepper in the mix.
The bigger question is, why would lesbians patronize a business where their deviant lifechoice is reviled? They're just asking for a loogie on their cheeseburgers.