Which BlackBerry devices do you have? In OS 4.2 and newer it is possible to turn off data connection and only have the phone working. For 4.1 there were only one release with version no. 309 which allowed this function. This release only exist for the 8700 and possibly the 7290. For the 8700 you can download it at http://www.phone-edition.ch/Download/309.exe.
Temporarily disabling the email account won't help. As long as your BB shows upper case GPRS there will be roaming charges.