Hypothetically (of course), I like the idea of taking the keyboard and keeping the keyboard in the custody of a responsible adult. Also, anything that would temporarily disable the monitor, like taking a connector cable with you when you leave the house, would be good as well
And, hypothetically, if you wanted to be really sneaky, I would make this mythic teenager promise not to use the computer and then disable it when they're not around.
The passwords are good, unless they really understand computers and have hacker tendencies.
Do they have a computer in their bedroom? And what they might be getting punished for is really major? Then maybe take their personal computer *away* & allow them to perhaps earn it back.
The Internet ban would be difficult in our house, mostly because so much of my DD's research for school projects/papers is done on the Internet plus the teachers often put assignments on their school homework system. When she gets home, DD has to sign in with her password and see what the assignments are.
Good Luck! Parenting isn't for sissies, is it...
agnes!