Yes, COMBO, which is a part of OMBA, is actually in charge of the mountain bike trails at alum, they built them, they maintain them, and they patrol them. If COMBO says the trail is closed, the trail is closed and if you get caught riding it when it is closed, you WILL be ticketed.
BTW, I am a member of COMBO, I suggest anyone who rides a lot to join, the dues money helps maintain the trails