Sounds to me like you need to make it well known that the MTB group is responsible for the trails, and the cleanliness/maintenance of the entire park. A lot of them are just self centered d-bags who probably don't even realize that those trails don't just occur naturally(who'd of thought?) and that your group is out there taking care of them. They might have to think to figure something like that out, and thinking is not a strong suit of the standard self centered d-bag.

I personally say post some signs at the beginning of each trail that say something like "You realize these are obviously MTB trails, right jackass? I thought so. Feel free to use them, but how about moving out of the way for MTBs, considering their club are the ones who put in these trails and maintain the entire park for you."

In light of the fact that you can't do that, some simple standard park signs will probably help your situation. Ours has the standard signs with a bike, a hiker, etc, at each trail head. I rarely see hikers on the trail, and they're always courteous and step off of the trail when they see me coming.