Or there is this solution:

The BIOS password is only good if they're trying to keep him off the computer altogether. If they want him on the computer but blocked from doing what they don't like, that won't help.

If it is a very specific thing they want to block then switch to a wireless router if needed, block the thing at the router level and put it somewhere it is possible to lock the kid out of.

That.

It's still not foolproof of course, but at a certain point one imagines it will just become easier to go somewhere for his illicit computing needs. I hear public librarians like things like that that.

Too bad we don't know any to get their opinion on that

They are trying to prevent him from using the computer. Temporary restriction thing. The "kid" is 18 and it sounds like the motto is now survive until HS graduation. Locking the BIOS or removing the machine are their best options.

Thanks for the actual suggestions.

They should encrypt their sex tapes and move them off-line

If they go the route of a bios password then be sure they know that they have to shutdown the computer each time they're done using it.

Get the son one of these:

Wow, if I ever have to lock something to prevent my 18 year old from using it... :(