There is no direct way to clear a https://community.bistudio.com/wiki/assignedVehicleRole
One has to use workarounds as leaveVehicle (or as player GL also unassignVehicle does it).

The idea to be able to assign vehicles roles and not having AI to enter the vehicle right away, or to have it persist when leaving a vehicle (ie temporarily) is very useful and offers great flexibility. However there should be a SQF command to control this directly.

unassignVehicleRole _unit

Overall management of using vehicles, getting in and out as desired/reliable, is still very complex to manage.