1. Refuse to order.
2. Asked to leave.
3. Don't leave.
4. Employees call the police.
Seems sensible to me. I don't understand the logic of saying "Yeah but as a white lass, I am never asked to leave." I would assume this bloke was not accustomed to being asked to leave either, hence his assuming he could go in and use the facilities without purchasing something. This is one store out of thousands and their circumstances could be different to your local; i.e. a petty manager who does this to everyone, or busy so they need the space. Just because it doesn't ordinarily happen with you doesn't mean it doesn't happen at all.
I am the whitest white person you will meet and our uni, even if there were 0 people in the restaurant, would force you to leave if you hadn't purchased anything. Me and all my white mates told to bugger off, every single time unless we bought a water or something along those lines. And guess what, it was a very white uni too. The inherent racism of white against whites! Must be!