If your stabilizer is as high as we think, it's not over chlorinated -- it's actually under chlorinated! (See Best Guess chart)

Sometimes biofims form in spa piping, and is very hard to remove. Sustained very high chlorine levels will do it. But your stabilizer level may be so high that's not practical. In that case, it would be best to do a high chlorine just after filling, before adding any stabilizer. 10 - 15 ppm with no stabilizer for 24 hours should do it. If you add stabilizer afterwards, those levels won't be too high to swim in.