https://support.pokemongo.nianticlabs.com/hc/en-us/requests/new?ticket_form_id=319948
I doubt anything will be done, they have a small staff, large work load, and lots of support tickets behind I'm sure.
Banning is a big step and has to be investigated if not obvious (obvious in other games with manipulated stats and items). All you can really do is provide their screen name and claim they cheat. Your best bet is just find another gym and hope they don't frequent it.
Good luck though.