Cancelling callbacks

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Hello, I am very new to ROS and am trying to make the system work in a very specific way. I want to create a Publisher/Service that will send multiple instructions that would be read by other nodes. Depending on the response I receive from one of these nodes I want to be able to stop the other ones. For example lets say I have a node that works with an ultrasonic sensor and another that works with the motor controls. I send the ultrasonic sensor node a message to report when the robot is a certain distance from the wall while I send the motor node a message to move randomly (meaning that it will not exit its callback). How do I make it so that when the ultrasonic node sends back data I can stop the motor node?

Also I am using rospy