If the pool contains a single VM it will always be powered on even if the Remote Machine Power Policy is set to "Take no power action".

Power Policies for Desktop Pools

Note:

When you add a machine to a manual pool, Horizon 7 powers on the machine to ensure that it is fully configured, even when you select the Power off or Take no power action power policy. After Horizon Agent is configured, it is marked as Ready, and the normal power-management settings for the pool apply.

For manual pools with machines that are managed by vCenter Server, Horizon 7 ensures that a spare machine is powered on so that users can connect to it. The spare machine is powered on no matter which power policy is in effect.