Towny Jail Help

PLEASE READ ALL OF THIS BEFORE ANSWERING!

So I'm mayor of a town on a towny server, I've established a jail plot, but whenever I send someone to the jail on my tests, they're able to:

• Tpa out. (There's a tpa pluggin, I don't think this can be fixed...)
  
• Break blocks (They are NOT supposed to be able to do this!)
  
• Place Blocks (That are also NOT supposed to be able to do this!)
  
• Just about anything else they would be able to do without having been jailed.
  

I'm starting to think that putting someone in jail is completely useless.
Here's what I've already done to try and fix this:

• /plot set perm off (did nothing)
  
• /plot set perm friend destroy off (did nothing)
  
• /plot set perm friend build off (did nothing)
  

A bit of help would very much be appreciated.