My kids are little so like a slap on the wrist.
I feel a little pain now can save from a lot of pain later on.
If a small kid does something that needs a harsher response, I'll take their hand, slowly pull-up the sleeve, and with a stern face give a little slap on the wrist. ( It's done almost in Slo-Mo )
It doesn't really hurt, it's the tense buildup and the fact that the child has gotten a "Potch" that will bring them to tears.
They know that they crossed a red line and usually don't do it again. ( Varies of course )