Agency that I had hired to manage my Facebook page has gone rogue and is not handing over the ownership of the page. I am still the admin can I get the ownership back without him having to sanction it?
If you are an Admin of the page you can remove other people from their roles and then you will be the only owner of the page.
From the Facebook help Center:
Remove Someone Who Has a Role
If you're an admin:
- Click Settings at the top of your Page.
- Click Page Roles in the left column.
- Click Edit next to the person you want to remove, and then click Remove.
- Click Confirm. You may need to enter your password.
You can always remove yourself from a Page, but if you're the Page's only admin, you'll need to add another admin first.
Know more about What are the different Page roles and what can they do?