I assume you would like to block pathing for only certain players, but I don't think you can do that.
I would make a trigger that checks for units entering region. If they belong to someone not allowed to enter the region, disable command for that unit and order it to leave the region. Check for units exiting region and (order the unit to stop, maybe) and enable command again. Might wanna add a check on the units exiting so that only units with command disabled are affected.
You could also force move (instantly) any units that enter the region.