I think all mountain bikes should be prohibited from walking/hiking trails. What I mean by "mountain bikes" is more of an attitude or world-view than a bicycle design. These are the "type A" personalities, whose motto is "faster, more, louder".
The woods (or any natural environment) is my church. It is sacred to me. If I plan on being in the woods, I walk, slowly, to absorb and appreciate the experience.., until a mountain biker comes along and challeneges me for the trail.
Get off the bike and walk. Appreciate the beauty and fragility of nature.