I found the below on another site for Blackberry users, but the ideas may help you to. Look at what other apps besides the email which may be connecting.
From:
http://community.spiceworks.com/topic/332401) Block data or internet from the server or network end e.g. global BES policies
2) Properly close any application after use by clicking on the menu and selecting exit. Letting programs run in the background may use data and will reduce battery life
3) Don’t download any automatic updating programs e.g. stock feeds, weather apps
4) Don’t tether to a computer and use as a wireless modem (also referred to as DUN, dial up networking)
5) Enable WiFi mode (if available) and utilize free WiFi locations for surfing
6) When in WiFi – ensure you have not lost the WiFi connection and have accidentally switched to regular cellular coverage. One recommendation is to turn off the cellular radio when in WiFi so your guaranteed no extra data fees
7) Save unnecessary web surfing for a regular desktop/laptop

Set up email filters to keep unwanted messages
9) Use WAP surfing rather then full HTML surfing
10) Remove/hide built in icons/applications which offer pay services e.g. iPhone apps store
11) Ensure smaller or simpler applications are used which are lower data consummators e.g Blackberry Maps uses 5 to 10 times less data then Google Maps
12) Use basic GPS & Maps rather then services like TeleNav which will consume more data
13) Limit the downloading of attachments like Word or Excel files for important and time sensitive reasons only
14) Choose “view only” rather then downloading a full attachment if that option is available
15) Do not surf to streaming video sites e.g. YouTube a rough/general rule of thumb is 1 min. of video equals 1 MB of data
16) Do not enable remote video viewing services/applications e.g. Sling box
17) Do not attach & send files from a device unless the attachment is residing on a remote server and you are just sending a command to send from the server rather then sending the whole document from the device. e.g. send via BIS rather then sending a Documents to Go file on the new Blackberries
18) Download applications through your desktop software rather then OTA (Over the Air)
19) Know your service plan and use other means of communication which may be part of your plan or free to use e.g. SMS, MMS, Pin to Pin, voice calls, emails
20) Have a meeting or 1 on 1 discussion with end users to inform/train and ensure understanding of how mobile data devices and services work and are charged.