Can a family member with POA allow you to prevent others in family from visiting an elderly parent?

If the parent is competent to make their own decisions, then no. The parents should be able to see who they want.

If the parent has a form of Dementia and has been found incompetent to make their own decisions...then its a thin line.

Does the persons presence cause anxiety in the parent? Is the parent living with the POA and the family member abuses the Caregiver in some way when they visit? Then I would say that the POA is protecting the parent and being their home, they have the right not to let someone in.

Caregiver living in parents home, this is the thin line. If family is being prevented from seeing a parent, in my opinion, the POA does not have the right if its for personal reasons. Like the POA can't stand a certain family member.

Too many people misinterpret a POA. Its a tool for the person who has been assigned. It helps when bills need to be paid. Selling of a house and other things. You are a representative of the Principle and personal feelings do not come into it. Your decisions should be made with "what would Mom/Dad want?" Some people it becomes a control thing. Some abuse the POA.

POAs can be immediate, meaning as soon as the Principle signs the POA is in effect. Then there is Springing which means for the POA to take over, the Principle has to be found incompetent.

If the person has POA for health care, and visits have been unhappy and disruptive, therefore detrimental to the health and well being of the elder, then the answer is a resounding yes, they can prevent such visits.