In a friendly setting, you can always just scold the MK after he nado spams too much and give him a lecture on banning MK.
In a tournament setting, you can punch the player while he is nadoing, and if the ref has a heart, he will take pity on you for having to fight the cheap ******* and pretend he didn't see anything.
Anyway, I'm not entirely sure exactly how useful PSC is, as there seem to be plenty of things about nado it can't prevent. What I would do is try for an USmash/BAir OoS, but it takes good timing and spacing. You can try to wait for an opening to jump/phantasm around them, and then (smartly) spam lasers to outprioritize further attempts. If they full out spam and use it 4-5 times in a row, then keep on them after knocking them down once and take advantage of nado's small startup lag until they figure out it isn't working and switch to a less gay tactic, like DAir camping or planking. On second thought, f*** it. Just punch them.