Lock / Protect Rows Past Due Date

I have a workbook list of activities that I am hoping will visualise the scheduling of routine maintenance tasks in my engineering department. The date of the most recent check is entered in row 6 and the interval in row 5. Hence the number 7 denotes a weekly check. The conditional format is such that the reds indicate that a check needs to be carried out. If the check is done on the scheduled day then a "Y" is entered into the square. This will then turn the square green. If "Y" is entered on any other day then the square turns orange to indicate the check is done, if not on time. The red squares do not change colour unless a yes is entered. This is designed to show the history of checks and give us the opportunity to show compliance to routines. What I need to do is lock the rows that have passed the current date, to prevent unscupulous modification. Thus the macro would look down column A and if the date is less than today() would lock the row. I have truncated the file as much as I can. A nice to have would be the facility for an adminstrator to go in (perhaps password protected) to make changes if necessary.