staff release plan

Staff management plan in PMBok talk about staff release plan for smooth transition of the resources on the project. Now towards the end of the project if resources are started looking for other projects as next assignment, what should project manager do.

PM network with other project managers to find the assignments? Or
PM inform human resource department to look into matter? Or
PM just simply talk to resource that it is his legal responsibility to finish the current work?

they all sound reasonable but what is the best option?

Thank you.