How can I stop PVC decking from squeaking?
Take up the flooring and install proper joists to be 8" on center.

At 12" on center your diagonal distance would have been just under 17".  Now with 16" on center your diagonal distance is 22.6".  That's way too long, and of course it's going to pull and squeak.  I'd be worried about long term safety as well.  Manufacturers do give recommended maximum spacings for a reason, after all...

Bracing it with 2x4"s which themselves are going to flex under the weight can't fix the problem.  You need to properly reinforce the structure with properly sized joists.  

Basically you went cheap on the foundation - now you're paying for it.