When in Edit Mode, scroll up on the page you want to change access right (or go into "All Properties" view), and click the "Manage" link next to the value in the "Visible to" field (at the top of the page). If necessary, uncheck "Inherit settings from parent item", uncheck all boxes in the row for the group, then click "Save".

When in Admin Mode, in the "Admin" tab, click the "Set Access Rights" link. Drill down to the page you want to change access rights. Uncheck "Inherit settings from parent item" if needed, uncheck all boxes in the row for the group, then click "Save".