Cancel command sent on VehicleCommand

I’m currently trying to control a simulated quadcopter on jmavsim, viaros2 and uXRCE-DDS, using vehicle_command messages. I’m able to e.g. takeoff, thus everything is working so far.

What I can’t understand is: how can I cancel a command?

From the VehicleCommandAck (UORB message) reference looks like it should be possible (the cancellation can be ack-ed), but I can’t find anything explaining this.