M.M.
So, personally, I think this is a little sign of independence, and you need to treat it like you would treat any other kind of tantrum.
My son did this on and off. (3.5)
He doesn't get a vote. It takes 1 minute, if that, to wash hair. I have a big bucket that I use to rinse the shampoo out. 2 bucketfulls and we're done.
Maybe I'm more of a "tough love" kind of mom...but it's one of those things that you just have to do. Suck it up, kiddo!