“Can you just stop it please?”

You're working on a group project for school. You're trying to work, but one of the group members (who you know well) is just making jokes and chatting instead of working. You say to him:

Can you just stop it please?

just (do something)

This is an order to do something. You're telling someone to do only what you ask, without discussing it or protesting. Some examples:

Just shut up.

Just listen!

stop it

This means "stop doing that". "Stop it" is a more common way of expressing this idea.

Can you (do something) please?

This is way of asking someone to do something in an angry or frustrated way.

Adding "please" to a request doesn't always make it more polite. In many situations, asking without "please" sounds more polite. People often use "please" after a request when they're annoyed.