Option Explicit
Const RangeToWatch = "A1:B4"  'диапазон, где должна работать защита

Private Sub Worksheet_Change(ByVal Target As Range)
    If Not Application.Intersect(Target, Range(RangeToWatch)) Is Nothing Then
        ActiveSheet.Unprotect
        
        If Not Target.Value = "" Then
            Target.Locked = True
        End If
        
        ActiveSheet.Protect DrawingObjects:=True, Contents:=True, Scenarios:=True
    End If
End Sub