...no offence, but threatning to beat a kid ... thats just bad...
I don't know. It depends how you say it. You can either say it in a really serious, menacing way, a happy, jokey way, or a combination of the two so they know you are serious but not mean.
I have (had) a teacher who can control her class amazingly. Common phrases of hers are: "If you do that again I'll smack you!" and if a student says sorry, she says "You wil be!". Now before you get on the phone to the child abuse agency, it is done with just the correct degree of jokiness that shows that she won't really carry out her threats, but there is that seriousness in her voice that shows that she won't take no (*ahem*) from nobody. Now that's perfect audience control.
As far as prop-grabbers go, if you store your things in a case or bag (especially if you can lock it or secure it with a clasp), you won't have a problem with people grabbing your props when you're not using them. As for those with enough guts to snatch your gimmicks while you're using them, that's just rude. If you can get your audience to sit down, whether at school or on a bench if in the street, they will feel less inclined to get up and examine your props. It's usually just "too much effort" to stand up.