Unfortunately, my young kids threw some mud into the spa, I didn't notice it for a good two weeks, and the mud has really worked its way into the plaster. I have brushed at it a lot, I even scrubbed at it a bit with some pumice stone, and the mud stains are very deeply set. It's almost like they've fused into the plaster.
I will try to show some pictures later, but basically it looks like spots of mud on the floor, but when you brush at them, they don't come off.
Is shocking the first thing to try?