Dude that does suck.
I've been where you are. The guy was so heavy-handed with the reverb, and saying all the typical, "I've been doing this for years, I know what I'm doing, blah blah blah..."
None of the other guys would speak up ( I didn't hire him), so finally I said, "Look dude, just because I don't know my way around a console doesn't mean I don't have good ears. When I compare this mix to anything else, it doesn't sound nearly as good. Besides, you work for us. We've all said we don't like the reverb, so take it out."
So finally, he relented, but told us it wouldn't sound nearly as good, plus he wasn't finished with it. I was still like, I don't care. Just give it to me w/o all that goo.
I took it right out to my car, the guys got in, and we all agreed it sounded 1000% better. In fact, it didn't even need much polishing, especially since we weren't recording a fucking album, just a demo tape to get gigs. Which for that purpose, it did the job just fine.
I have also been in a later session where I made it clear up-front that I wanted to be responsible for the mix levels, but I'd need help with the finishing touches and such, and that guy was fine with it. He was like, "I'll do it however you want. It's your money." He had great mics, and captured everything well, I did the basic mix levels, then he took it from there, and it came out sounding great.
Lots of bands have a hands-on approach with the mixing, and work alongside/with the engineer. But in my limited experience, you need to get that agreed upon before the session starts.
Sorry you're having to deal with this!