Tresspassers

Are they tenants--paying rent?--or guests--not paying rent? The procedure is different for each. Since you do not mention a lease or rent, we will assume they are guests.
A guest may be asked to leave at any time; they have no right to stay in a home once the owner or renter (or a person with a power of attorney from the owner or renter) tells them to go. You need to provide written notice (which should be sent some way you can prove delivery) to them demanding that they leave; they should be given some reasonable time to actually vacate--usually a week to 10 days. If they don't then go, you can file a lawsuit for "ejectment," which is eviction for nontenants. As guest, or people who do not own and were not renting the property, they don't really have any legal defense to ejectment, but you still have to go through the courts to remove them.
Ejectment actions can be technical in that if you make a mistake in the paperwork or court filing, the action will be dismissed without prejudice, which means you would have to start over again. Also, while you have a POA, which gives you legal authority to remove someone from your father's home, you are still not the actual owner (or renter, if you father is a tenant) yourself; however, one non-lawyer, even with with a POA, may not represent another person in court. For both these reasons, you need to retain an attorney to bring this action for you--you cannot actually bring it in court. A landlord-tenant attorney would be a good choice: they would know ejectment actions as well.