We have processed the payroll on 22apr2011 and the post processing for the payroll has been done on Tuesday 26-apr-2011
The pre payment date is 29-apr-2011. The details in the manage payroll Payments should be frozen and should not be edited by the employees till 29th of april but the page is editable now for the employees. Any help on this issue would be appreciated.

Why dont you run the payroll and prepayments one after another on the same day i.e. 26-Apr-2011.
You can do the BACS/NACHA/send file to the Bank at a later date.