Buy another helmet and rotate between the 2. I never put a helmet that is still sweaty and wet on the next day. A couple of years ago, I was having terrible problems with my head itching; it happened only after a ride where my helmet was not completely dry from the day before. I did try wiping the helmet down, washing, etc.
Now, when I finish a ride, I let my helmet air out after giving it the sniff test. If it's not dry the next day, it's time to wear the other one.