Can you re-rent room if tenant vacates but paid full month rent?

[Becky Hiu]

Hello. I’ve an interesting situation going on where I’m a landlord and housemate to 4 other girls. Utilities are split among us. I served a non-renewal to one of the tenants (aka my housemate who was a difficult tenant) before the 30 day window and she moved everything out of her room mid-month. She’s already paid for the entire month of rent. After she left, she held on to the key and said no one can enter her now vacated room until month end as she’s already paid for it. Also, she said she isn’t liable for utilities used between now and month end as she’s no longer here.

Questions: 

1. Do I need to refund the portion where she’s not technically living here anymore, I.e. half month?

2. Can I re-rent her room out if I don’t do 1. above, or if I must do 1. above, I could just re-rent her room, right?

3. Is she liable for utilities until month end despite not having a bodily presence here any longer but is still holding on to the house key?

Any thoughts/direction on landlord-tenant law for a primary residence would be most appreciated.