I guess it depends what you mean by not acceptable. For me, the first step is thinking about why I think something Sam is doing is unacceptable. Is it dangerous, annoying, embarrasing, makes work for me (to clean up or whatever)? Dangerous, well you just have to deal with that. I experiment with different things. I'll say no, I'll explain why, I'll work at doing a redirect. Depending on how dangerous it is, I may let him experiment, which brings up consequences. For me, the idea of contrived consequences doesn't work. I think life has plenty of natural consequences. I've let Sam touch the stove when it was hot, but not in a way I knew he'd get burned. So he learned about hot. I've let him put his hand in hot water that wasn't hot enough to scald. I've let him tear up a pop up book to see how it works and gave him hugs and kisses when he realized we couldn't put it back together again.

If I want him to stop doing something for other reasons, I have to really figure out how important that is to me. So far, when I feel like we need to change some habit we've gotten into, I work really hard at finding a path for us to move forward in a good, nonpunitive way (I don't always find it, sometimes I'm just grumpy). I feel like when I start focusing on something he's doing and trying to force him to stop, we really get stuck there. If I stop giving it all my energy, it really goes away. For instance, this morning he was bored and wanted to fling things around. I had this idea we'd spend some time sorting a new haul of embroidery thread I got at the thrift store. He just wanted to fling it because he needed to move his body and do something exciting (some days he would happily sort thread into different piles and stuff it in bags for an hour). So I don't try to eradicate his habit if flinging things, I try to make sure he gets all the movement, interaction, and attention he needs.

I guess, I could have given the short version which is, if he's doing something I don't like, I try to figure out why and address the whys (why is he doing it, and why does it bother me), which pretty much always resolves the issue.

Also what might be acceptable at one age can quickly become unacceptable at another. Helping them to understand the finer nuances is an important process.