So I set a group policy up to only permit use up to 9pm. The device I have allocated the policy to is still be used after 9pm, in fact it is 10.16pm and it is still in use. Why?
If you have configured a group policy to work up until 9pm then at that time the group policy will stop applying and whatever other rules you have will take affect.
If those other rules allow access then you'll still have access.