Locking last month timesheets in Planview Portfolio?

Hi All, 

I am looking for you expertise - please help.
We have weekly timesheet time periods. We submit and automatically approve our timesheet on Friday CoP or on the last day of the month. Progression runs automatically every Sunday night and at the end of month.

Finance consolidates the timesheet actuals at the end of each month (extracting data from Planview).
We want to lock timesheets so that they can no longer be disapproved/edited once the month is closed.
We can disapprove/edit timesheets for the current and future months but not for last month.

How can I do this in Portfolio please???
How to you lock timesheet / cut off period etc

I look forward to your help on this.
Many thanks in advance

Tony.