Lock Past Rows As I Trigger A New One To Appear

This (unprotects the sheet first, then) unhids rows (starting at row 1500, until it reaches row 4). Now my most current vexation- How do I lock/protect the past rows?
Say a co-worker enters information in row 1500 and walks away. The next co-worker inserts a new row (which would be row 1499) to place data in there, how can I get row 1500 to lock (or become protected) as the new row (which in this case 1499) would become unhidden?
It would be very important that the information in the previous rows remain locked so that nobody can change any client information (unless they had supervisory/administration access).

If it is not possible to protect or lock the whole row, it would also work if I could just lock one of the cells in the previous row.

I am using the following

Sub Row_Unhiding()
'
' Row_Unhiding Macro
' Macro recorded 2/12/2009 by gr8_big_geek
'
' Keyboard Shortcut: Ctrl+i
'
ActiveSheet.Unprotect Password:="secret"
R = 1500
Do Until Rows(R).Hidden = True
R = R - 1
If R < 4 Then
MsgBox "No more rows to add!", vbCritical, "Error"
Exit Sub
End If
Loop
Rows(R).Hidden = False
ActiveSheet.Protect Password:="secret"
End Sub