There's two different policy settings under the product policy: there's "Enabled" under the "General" tab that controls whether EEPC activates or deactivates, then there's the encryption policy under the "Encryption" tab that controls what disks/partitions get encrypted, when the system is active.
Sounds to me like you want to deactivate. So you can navigate to the "General" tab, uncheck the "Enabled" policy setting, save the policy, and then trigger a policy enforcement on the client. You won't need to touch the Encryption settings ; if the policy isn't "Enabled", all drives get decrypted, and then pre-boot is removed.
Hope that helps!