Undo Action In VBA

I have a database in excel which I want only selected users to have edit rights & others should have readonly rights. I have written the following code where a pop up would appear in selected cells warning user for editing the said cell. What I want is when a user clicks yes he should be able to edit it & when he clicks no the cell should get protected.

Private Sub Workbook_SheetSelectionChange(ByVal Sh As Object, ByVal Target As Range)
Dim rTriggerCell As Range
If Not Intersect(Target, Range("D1:D100")) Is Nothing Then
Set rTriggerCell = Target
Application.EnableEvents = True
If MsgBox("Edit Cell?", vbYesNo)
End If
On Error Goto 0
Exit Sub
End If