Unless it's already roaming, it's trapped in it's skull regardless.
If it is a ghost, a circular ward around the skull, use Rego to force the ghost into the circle, then Whispers when you want to talk to it. Or put it in a Whisper with ring duration.
This is in no way the practical approach but I do feel like enchanting the skull such that if you throw it at the ghost and it hits the ghost is now trapped would be more fun. Maybe the skull wiggles for a few seconds while you wait to see if the ghost was caught.