The only option you can find related to Bitlocker is "Require" or "Not configured".
You need to create a custom OMA-URI policy to enable the setting "Security/PreventAutomaticDeviceEncryptionForAzureADJoinedDevices" in Policy CSP.
Here are some posts with the similar issue with yours, just for your reference, you can try the method mentioned by Mark_Thomas:
https://social.msdn.microsoft.com/Forums/en-US/d9ff2e01-5e36-4aa2-8a92-31094502e1da/can-we-disable-automatic-bitlocker-encryption-on-azure-ad-join?forum=WindowsAzureAD
Hope above information can help you.
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