“What are you doing? I was totally looking when you hit him! Don’t you know that you’re supposed to wait until the teacher isn’t looking before you hit him?”
“I know you want to spend time with me at recess, but you don’t have to talk while I’m talking – you can just say, ‘Please Ms. Yuill, you’re a great substitute teacher, may I spend time with you at recess?’”
“No hugging allowed in class.” (When 2 boys are wrestling.)
“Please wait until after school to hurt yourself.” (When students are playing with scissors etc.)
These statements all get better results than simply telling an intermediate student to stop doing what they’re doing. If you can get them to laugh, they’re much more likely to do what you’re asking.