If you have granted "Timesheets - Approve All" Global access rights to the resource Managers directly or using Group the Resource managers will see all the Resources Timesheet in the application.
Remove the "Timesheets - Approve All" access right from Global Resource Access Right's.
Add "Resource - Approve Time" access from OBS Units for the Resource Managers at Resource level or Group level where the Resources will displayed based on the OBS Units Selected.
or
Add "Resource - Approve Time" Instance level access to the Resource Manager and add the respective resources that you want to approve Time sheets.