how do you do a master reset on an iphone 3Gs after its been locked out saying disabled?

Locked Out, Forgot Lock or Restrictions Passcode, or Need to Restore Your Device

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Forgotten Restrictions Passcode Help

You will need to restore your device as New to remove a Restrictions passcode. Go through the normal process to restore your device, but when you see the options to restore as New or from a backup, be sure to choose New.

Also, see iTunes- Restoring iOS software.

Follow the instructions in this support document http://support.apple.com/kb/HT1212. You will have to connect to the computer that you normally sync to, restore the phone, then restore your latest backup.