You wish to prevent any modifications to timesheets because it is ready to be exported? Here's how to close or finalize a pay period.
Via the "Attendance" tab
When you finish approving timesheets and that the pay period is ready to be exported, it's recommended to finalize the pay period for two reasons;
To prevent modifications from other individuals
To protect the dates included in the pay period if you ever change the frequency of your future pay periods
Note : Only an Administrator can modify the status of a pay period (Open, Closed, Finalized)
To do so, you can simply click on the status next to the pay period.
If you want to modify a pay period that is already finalized, see this article!