Remove custom policy in Skype for Business Online

Dear Skypers,

I created a custom Desktop Conferencing policy. Only one user is assigned to it. Now I don't seem to be able to remove this custom policy. I tried:

Remove-CsConferencingPolicy -Identity CustomDesktopConferencingPolicy -Force 

Result: The policy "CustomDesktopConferencingPolicy" is currently assigned to one or more users. Assign a different policy to the users before removing this one. 

Grant-CsConferencingPolicy -Identity "{identity}" -PolicyName $Null

result This policy must be assigned a value compatible with the user's subscription and location. It cannot be $null.

Can someone explain how to remove the custom policy? Many thanks in advance!