See the article on removing an IT Policy at blackberryfaq.co. Remove IT Policy - BlackBerryFAQ
According to the article, process involves a security wipe.
Unless you have to remove an IT Policy, you may prefer to do a security wipe from the security options screen. How to wipe ALL personal data from a BlackBerry device? - BlackBerryFAQ
(The articles may be old, but the concepts are the same.)
If encryption is enabled on the device memory, a security wipe will perform a very secure wipe. Otherwise, I'm not sure the security does any more than delete the data, but doesn't repeatedly overwrite the memory.
No advice on best way to do many, many devices. See what others say. You may have to figure it out yourself. Posted via BlackBerryForums.com Mobile