Dynamic Macro To Lock Cells That Contain Any Value

I'm currently using this macro:

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
Dim Rng
Dim MyCell
Set Rng = Range("A1:AA7672")
For Each MyCell In Rng

[Code] ....

The problem is that it is not dynamic and it's super slow. The larger my range is the slower it operates and right now it's almost unusable.

The workbook I am using gets touched by 10 different people and I want to prevent tampering. I want for the workbook to be, when protected, have all the blank cells editable and once a value is inserted, it cannot be edited again and not work so unbelievably slow.