Until I came to this thread it didn't hit me that someone who buys my old prevail (or any boost phone really) can get all my information off the phone!
My phone number, my msid, my shared password (not so much since its the same across the board), my aaa password... Good thing I haven't sold any of my phones yet.
But to answer your question. You have to know your msl. You have to be in an epst compatible rom. At the moment any rom but cm7.
Open the dialer and enter ##yourmsl#. Then you'll be asked for your msl again. Enter it and press OK. Once pressed you'll be presented at the Service Menu. You want to click/touch Edit. You'll be presented with your phone number and the keyboard will be at the bottom. You can edit out your phone number and then press the Enter button on the keypad (last button on the lower right hand corner).
Then you'll be presented with the MSID. Edit it in the same fashion and press the Enter button again. All zeros are not going to be compatible. Once you get into the Service Menu again press/touch Done.
The phone will reboot. If the phone didn't reboot by itself you didn't do something right.
Please note. The phone number and the MSID and the MEID of the phones have to match in order for someone to "steal" your service. Scrambling one, the phone number or the MSID is enough to derail stealing service attempts.
The phone number and the MSID in the Service Menu have to be 10 digits each. In order to put in "dummy" information you can just put in (areacode)555(any4digits). Or just use your imagination!
Good luck.