###“You want the spice? You can't handle the spice!”
That's what typical Asian chefs think about westerners in their restaurants.
It's certainly bad to discriminate. But this isn't discrimination per se. They consider that it's better to play it safe, rather than upset the customers' stomachs and later watch them complain. It's well-known that westerners, unlike Asians are not the best consumers of spicy foods, and are therefore ill-equipped to deal with the probable aftermaths.
I'm an Indian, and I've eaten spicy food all my life. And I don't think any western restaurants I've been to have ever served me anything as spicy. It's just the way it is.
All you can try is ask them nicely as you usually do, or maybe:
Please add all the spice you've got. I'd like it to be extra extra spicy.
And hope that works. There's no guaranteed method for this.
Or ask your friends to order for you, if they're Asians. Maybe that would help.