How to "Virtually" Ground a Teenager
blogs.wsj.com
blogs.wsj.com
How long until she finds out that you can just reset the password as long as she has email access?
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).
(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.