I'm gonna have to cut off my kid's interne access for breaking our "contract".
What were the terms of this contract they agreed to?
My son agreed to attend jiu-jitsu classes twice a week if he can have regular internet access.
It's okay to punish a child who broke rules they agreed to follow. They may be upset but they'll learn that rules should be followed if they're willing to agree to them.
He broke his promise and now really needs to learn that his actions have consequences.
Then that's fair. This can be a good, teachable moment for you and your son.