As one who appreciates B-C solitude I have told people on ATV's that they were on a restricted, walking-only trail and politely asked them to turn around. The 1st ATV guys sneered, told me I wasn't a ranger and roared on.
The next time the group was more adult and respectfully apologised and turned around.
As a Nordic ski patrollar I had to chase renegade snowmobilers off our trails as well as skiers with dogs (!). Then I had the authority and had to use it, but always with courtesy, or when that failed, resolute firmness.
I believe you should politely inform people of their "misbehavior". Then, if they persist in it you know they are acting not out of ignorance but stupidity.
