You might want to post this in the main forum - this is for matters regarding the WA kitesurfing association only and not many of the peeps bother to read it.

I, too, have bad reactions. Other then wearing a full suit, the best thing to do is avoid calm water. They don't like swell or wind chop. They come-and-go at various times, too, and you have probably just had a bad week. Funnily enough, those three spots you mentioned are a bit notorious for them and it must be the conditions of those spots that concentrate them, maybe the prevailing currents. I have never been stung at Leighton, Port, Cottesloe and Scarb/Triggs but have been at all three of your spots.

Cheers.