Assuming here that you have created your own view to create this ad hoc report? This is the option in Ad hoc that may help you. In building the view (I think the option exists still in the view or report edit too)-- go to Pre-Filters
set the status to posted (as timesheet status would be in your filters)
set this to locked
This is supposed to lock the parameters so that end users cannot change the values-- it has been a while since I played with it, but that is what I recall. Give it a try.