I do it all the time. The people currently living with me have a 2 year old named Ashton. He throws tantrums daily with them. Not when Im there to step in. I have had to spank the kid only once and thats all it took. He even starts to think about having a tantrum I get my mean look on and ask him in my angry voice if he wants his butt to be spanked. He says no and quiets down. End of story.
Im still waiting for him to test me again, I will spank him again. BTW his parents gave me permission and no Im not wailing like a mad man using full strength or anything. Despite not having kids of my own I DO know how to handle them.
For those of you parents with a terrible 2, just try it, get a friend you can trust and give them permission to punish the kid. The kids are smart enough to know they can get away with from mommy and daddy, but when its a complete stranger (well, not a complete stranger, someone not related, you know what I mean) they WILL show respect if you demand it properly.
Parents today are just scarred of being the "bad guy" with their kids. End of story.