Two ways.
If no User should send commands, untick iot_rpc.send.basic for User on the Access Rights tab.
If it’s only the contractors, put them in a group (key contractors, say) and add a Deny access policy: actions iot_rpc.send.basic, iot_rpc.send.privileged and iot_device.attributes.write, condition subject.roles contains contractors. A deny wins over any permit. Policies need the Owner role. Turn on shadow mode first and watch the decision log for a few days before you enforce it.