I dont know of any international signals on this one...
But I do know, that what we always intuitively do works - almost everyone I have seen do it this way.
It happens quite often in fact, that some have to abort, so I dont agree with those saying it hardly never happens.
Crossed lines, something wrong with your shackle or QR or whatever, bridle wrapped quite common, tip snagged (foil kites), or simply bailing out in the last second as you find it is too windy, or can see a squall coming, or the safety line is not right - lots of reasons why you DONT want to launch...
Could not find pictures on the net, so went in the garden with my mobile, no sound but illustrates that you yell to the helper in a language that is understood.
would fit most
Above works splendidly
Tapping on your head can be way too risky, as raising one arm (thumbs or not, can not be seen if raining or sand blowing) is the signal to let go and can easily be mistaken
One can always discuss whether it is better to hold the bar if the helper let go anyways, or hold the QR, but in both cases when you raise one hand only, the risk of misunderstanding that it means something is wrong and letting go instead is much higher IMO.
Waving arms low across, maybe walking towards the helper so slack lines, works - and when you can see the helper lowers the kite you can tap on your head to show it is not something that can be fixed now, the kite should be put down (ha haa, sounds like it shold be "shot dead" - but maybe it should if something is wrong
Above signal for stop/abort can be understood and not mistaken for an arm up, even for dummies or anyone not into kitesurfing IMO and experience.