Children can be stubborn, even more so than adults. My sister's kids are little rascals. They do things that can endanger their lives at times, like trying to unfasten their seatbelts or putting dangerous objects into their mouth. And it is hard to strike a balance of being a good parent/carer but being firm at the same time.
But smacking them is not the answer. Simply explain to them, with love, the consequences of what they're doing. Love is the answer. Lots of love. I've seen it for myself.
I will never accept smacking children.
If smacking worked, then it would only need to be done once. Clearly that isn't the case, as kids will be kids. So... next time you have to smack them harder. And harder. And before you know it... you will be beating your self up for having thrown "pots and pans" and shoes and phones at them.