How do I block Timesheets from being modified?

1. Go to Setup under Preferences in the Sidebar.
2. Click on Timesheet Preferences.
3. Scroll down to Block Options.
4. Click on Set Rules.
5. In the popup, you can set restrictions as required. 
6. The admin can also reject a timesheet submitted for the blocked period. To enable that, check the box Allow admin to reject timesheet for blocked period.