To remove an admin from your page, visit your page and visit the "Settings".
Go to the "Page Admins" section and there you will see all the admins on this page.
The other admins will have an "Edit" link near their name. Click this and a dropdown will show up.
There will be a "Remove Admin" option. Click this and you will be prompted to confirm that you want to remove this admin. Once you confirm, they will be removed from your page.