Do you know how to cancel a turbo zap when it is pending and flashing?

That should mean you have zap undos enabled in your settings. The help text mentions this for them:

• After every zap that exceeds or is equal to the threshold, the bolt will pulse
• You can undo the zap if you click the bolt while it's pulsing
• The bolt will pulse for 5 seconds

So @Coinsreporter was right.