After applying a policy to a policy target rule within a device group, the policy cannot be removed from the child group's policy target rule.
The policy within a policy target rule group is marked as inherited in the applied column
While a policy cannot be removed when inherited from the parent in a policy target rule, it can be overridden.
To override the inherited policy, apply the desired policy directly to the device group, selecting the desired policy target rule and confirm you want to override the policy.
In the screenshots below the System Policy for Laptop Policy Target rule is applied and inherited by the Laptop Policy Target Rule.
A policy, in this case the Default System Policy can be applied to the device group overriding the inherited policy and is applied directly to the device groups policy.