Using SmartMonkey Planner you can assign zones to your vehicles so that those stops that are within that space are visited only by the drivers that you decide.
Once you have your zones created, all you have to do is assign them to your drivers. You can do this procedure from Assets (and they will be assigned to all your future plans as long as you do not modify it), or within the plan (only for a specific one).
In both cases the steps you should follow are the same:
Double click on the vehicle.
In Zones, assign the zone(s) from the drop-down list.