Mac Geekery

i m the mom of the youngest of 3, a 17 yr old who is cracking all my password efforts.
i would change the password for his account sign on, as well as my own password, to
control how much he is on my imac when he does not do well in school. over the last
couple of days, i changed the passwords twice and found him on anyway. when asking
him how he is doing this, his reply is “i know at least 3 ways to get around your password
changes”. heck i don’t and i thought if the admin changes an accounts password, then
locks up the changes, that was that. any suggests to really lock him out when i need to?