When exporting time data, you can choose to Mark the data as EXTRACTED (to effectively freeze a timesheet from further change).  

When exporting time data and using the Mark as Extracted option, any timesheets that meet the export selection criteria will be marked as extracted regardless of additional filtering that may occur due to expressions such as the !IGNORERECORD! expression.  That is, if a timesheet was included in the processing but subsequently filtered out due to an export expression, the entire timesheet will still be marked as extracted.

Note that when you check this option, several other selection criteria options have their choices limited, including;
Date Range requires a time period, you can no longer query by project, and certain statuses become unavailable.  See help on those particular selection criteria options for additional information.