You can use the Time Tracker Approval process so that Managers can Approve timesheets for the Users that report to them. You can set up te system such that once the timesheet has been approved, the user can no longer Edit that Timesheet Detail. You can do this in multiple ways:

Using Time Review

  • Set up your Time Tracker tabs, such that your Users DO NOT have the Timesheet Details tab available to them. They can then use ONLY the Time Review tab
  • In the Time Review tab, Users only see Timesheet Details that are in the Pending Approval status. Once a Timesheet Detail has been Approved by a Manager, that Timesheet Detail will no longer appear in the Time Review tab. And therefore the User will not be able to Edit Timesheet Details that have been Approved by a Manager. 

See the screen below for the tabs described here.

Use a Workflow rule

Set up a Workflow rule on the Timesheet Detail object. Set it up such that once the status on the Timesheet Detail record is changed to "Approved" by the Manager Approval process, you make the Timesheet Detail record "Read Only". In this case, the User will still be able to see the Approved Timesheet Detail record. But they will not be able to modify it.