IMO, proper response to the testicle slap would be to speak in a loud but calm voice, saying "I NEED A SUPERVISOR AND AN ACTUAL POLICE OFFICER OVER HERE *RIGHT NOW*." Explain exactly what occurred once both have arrived and press freaking charges for assault. They are not NOT *NOT* entitled to commit violent physical assault in the course of their duties; deliberately causing pain should carry legal consequences.

Calmly advising the screener who committed the assault that if it is repeated, he'll be laid out on the floor, may or may not be a good idea. Perfectly understandable, of course, but you don't want to end up in jail when it's the screener who actually belongs there.