It is easy for people to make a judgement when they are on the outside looking in. This is especially true when they do not know your child, or the way that your child reacts to discipline or punishment. Each parent individualizes their discipline to each of their kids. They know what their child reacts positively to, and what they do not respond to at all. They realize that cookie cutter punishments do not work on all children, which is the case with my children. Since my children have ADHD, I don't punish them, I redirect them.
People Who Are Clueless:
People who do not know my children as individual people, or the way they react to punishment or redirection assume that if I yell at my children, they will stop "running a muck." Little do they know, my kids would only become more hyper from the excessive stimulation that yelling at them causes. They do not understand how the ADHD mind works.
How I Handle These People:
The only way to really handle people who give unsolicited parenting advice is to smile, thank them, and go on with what you were already doing. It sounds like you are giving in, but in all honesty, you won't get any further arguing with them, than you would following their parenting advice.
Sadly, most of the people who want to tell you how to raise your kids, do not have a single child, nor have they been in the room with a child for more than five minutes. This even goes for social service workers. Yes, I really said it. The toughest of social services workers do not have any children, and if you ask them, they willingly admit so.
Essentially, you should take unsolicited parenting advice with a grain of salt. It isn't worth much to you, but it made them feel better that they actually "knew" something today.