I further updated the code to protect cells in the worksheet by clearance in a better manner. Some cells are protected by data validation, so far cannot be cleared by "backspace" but still when pressing "Delete".



Option Explicit


Private Sub Workbook_SheetChange(ByVal Sh As Object, ByVal Target As Range) 'Special Sub which is triggered whenever a cell value is changed
  
    Dim UndoList As String      
    Dim result As Boolean       
    Dim row As Integer          
    Dim col As Integer          
    Set Target = ActiveCell     
    
    On Error GoTo Terminate    


    With Application            
        .EnableEvents = False   
        .ScreenUpdating = False     
    End With


    If ActiveSheet.CodeName = "Sheet1" Then     
        row = 15
        col = 19
    ElseIf ActiveSheet.CodeName = "Sheet2" Then
        row = 16
        col = 17
    ElseIf ActiveSheet.CodeName = "Sheet3" Then
        row = 18
        col = 19
    End If
    
    UndoList = Application.CommandBars("Standard").Controls("&Undo").List(1)    
    
    result = IIf((Target.Column >= 1 And (Target.Column <= col And Target.row >= 1 And Target.row <= row)), True, False)    
            
    If result = True And Left(UndoList, 5) = "Paste" Then       
        MsgBox "Please don't paste values in this area of the worksheet."   
        With Application                
            .Undo                      
            .CutCopyMode = False        
        End With
        Target.Select              


        ElseIf result = False And Left(UndoList, 5) = "Paste" Then      
            Application.Undo              
            Target.Select                  
            Target.PasteSpecial Paste:=xlPasteValues, Operation:=xlNone, SkipBlanks:=False, Transpose:=False   
            Union(Target, Selection).Select     
    
        ElseIf UndoList = "Auto Fill" Then     
            MsgBox "Please don't use 'Auto Fill' in this workbook." 
            With Application          
                .Undo                   
                .CutCopyMode = False    
            End With
            Union(Target, Selection).Select     
            
        ElseIf UndoList = "Drag and Drop" Then     
            MsgBox "Please don't use 'Drag & Drop' in this workbook."   
            With Application       
                .CellDragAndDrop = False    
                .CutCopyMode = False        
                .Undo                       
            End With
            Union(Target, Selection).Select     
        
        ElseIf UndoList = "Clear" Then   
            MsgBox "Please don't 'Clear' cells filled with values in blue font in this area of the worksheet."    
            Application.Undo  
            Union(Target, Selection).Select     
            
    End If
    
Terminate:
    If Err Then      
        Debug.Print "Error", Err.Number, Err.Description   
        Err.Clear             
    End If


    With Application           
        .EnableEvents = True   
        .ScreenUpdating = True  
        .CellDragAndDrop = True
    End With
End Sub

Cheers,
RobGER