killTimer(which number)
From pinHeck Wiki
Terminates a number that has the numberStay bit set.
Example:
killTimer(2);
Terminates persistent number object #2.
Usually you'll want to make a number persistent for use as a countdown or hurry-up timer. Any number with the numberStay bit set will remain on the display until killTimer(x) is called.
killTimer(x) would normally be called when a hurry-up condition is met, the timer times out, or a ball drains terminating the hurry-up. Be sure to keep track of which object # the persistent timer is, because you'll need to specify this when calling killTimer(x)