Restricting/Disabling imessage capability on my teen's iphone
You can disable iMessages by having your teen go to settings and making all the changes she wants to the following items:


iCloud
Mail, Contacts, Calendars
Messages
Facetime
Twitter, Facebook, Flickr and Vimeo


Then you will want to observe that she turns off iMessages on the device and then lets you turn on restrictions with a PIN that you keep confidential from the teen.

In restrictions, under general in the settings app, you will want to prevent changes to Accounts about 2/3 of the way down the screen under ALLOW CHANGES text. Select "Don't Allow Changes" and then check that iMessages is now dimmed out in the settings app.

Now, that will restrict that one app - however there are hundreds of app store apps that allow chatting and you might have to consider also preventing app store installs entirely to mitigate that risk. However, with phones and iPods getting cheaper and cheaper, if a teen wants to communicate, it's going to be hard to prevent that with policies and technological efforts that are more geared for toddlers and pre-teens.

Good luck reaching some balance of control and trust going forward.