Call them up and ask them for something, making statements about how you love Trump, etc.
When they refuse you service, sue.
Record the phone call of course.
This is blatant discrimination, the bakers refusing gays is precedent.
It’s tempting, though better I’d think to take out a large ad in a major local paper pointing out that “well, we COULD sue... but we’d really rather just let the market do to you what it will.”
I have to investigate the product so I know what to ask for.
The recorded part sounds great :)