> The only way to do it is to force her to change her password. That meant using LastPass to generate a 12 character password of upper and lower case letters, numerals and non-alpha-numeric characters. It had to be complex enough so she would not remember it.
How long until she finds out that you can just reset the password as long as she has email access?
Don't fall in to the trap of thinking this is a purely technological problem. What really stops a grounded teenager from still going out with friends instead of coming home after school? Nothing physical, nothing technological. This is human stuff, not computer stuff.
(No, seriously, take a moment to ask yourself that question and ponder the answer. It's actually a rich, interesting question.)
The real point here would be to raise the threshold of temptation high enough that it can be resisted, rather than strict technological restriction.
I think the best solution is to simply take the computer away. Like you said, that's group punishment, but there's a reason that group punishment is so effective. When I was a teenager, I didn't care what my parents thought about me, and so if all they did was ground me, I would just focus on getting through it and making sure I didn't get caught again.
But when you use the punishment to indirectly alienate the child from his/her peers, that is much more effective. Then they have to face not only your anger (which they don't really care about), but also their peers' anger (which they do care about).
How long until she finds out that you can just reset the password as long as she has email access?