The reason why a griefer prisson won't work it becuase...
1)you can simply disconnect then reconnect and you out of jail.
2) to succesfully do this it would take alot of code and work to make it possible, simply becuase it would have to be a auto-teleport to prison when they reconnect.
3) how is sitting in prison a punishment all you would do is go to a different server until the prison time ends.
4) how does this stop griefers from not coming back and griefing
and finally
5) if you did mannage to get a code working and have a way to lock the person to the server until there time is up. how would you keep it from being abussed by admins that just don't like people or want to mess with someone and miss type the command???
these are the reasons why I personally don't like the idea of a griefer jail.
but it would be halarious to watch them build up in the jail.
(ALSO
Oh MY Gosh!Oh MY Gosh!Oh MY Gosh!Oh MY Gosh!Oh MY Gosh!Oh MY Gosh!Oh MY Gosh!Oh MY Gosh!Oh MY Gosh!
LoL rainbow-dash your so silly.)