Can I interrupt the UR5 movement/trajectory?

Hello, I am working with an older Version of the UR5 with CB1 Version 1.5.7849 and ROS Indigo. I would like to perform a continuous move of one axis in one direction as long as I have a signal from a sensor. When the signal turns off, the robot should stop. My problem with this is, that ROS always (?) requires an end-point of the trajectory. To handle that problem, I thought about performing a 360° spin of that one axis and stop the robot during execution of the path. Can I do that? Like to remove the last X waypoints of the trajectory? Something like a safety stop, but without turning the robot in safety mode. Thanks in advance, Veit