Trapping [BACKSPACE] (subclassing)

I'm subclassing a richtextbox so I can replace some keypresses with others (like changing a TAB to four spaces). My problem is that I cannot prevent the backspace character from reaching the richtextbox. I have succeeded in preventing the TAB key, that works fine.

When the backspace key is pressed, three messages are sent to the window:
WM_KEYDOWN
WM_KEYUP
WM_CHAR

I have my code blocking those at the moment, same method as blocking the TAB key, but to no effect. Is there something different about the backspace message? My code is below:

VB Code:

Dim WasProcessed As Boolean
   WasProcessed = False
   
    Dim StartSel As Long, Temp As String, PrevLineSpaces As Integer, PrevLineTabs As Integer
    StartSel = frmMain.txtDocument.SelStart
    PrevLineSpaces = 0
    PrevLineTabs = 0
   
    Select Case Msg
        Case WM_KEYDOWN
            Select Case wp
                Case 8 '[BACKSPACE] is pressed
                    If frmMain.TabsAsSpaces = True Then
                    
                    End If
                
                Case 9 '[TAB] Is Pressed
                    If frmMain.TabsAsSpaces Then
                        WasProcessed = True
                        frmMain.txtDocument.Text = Mid(frmMain.txtDocument.Text, 1, StartSel) & String(frmMain.AmountOfSpacePerTab, " ") & Mid(frmMain.txtDocument.Text, StartSel + 1)
                        frmMain.txtDocument.SelStart = StartSel + frmMain.AmountOfSpacePerTab
                    End If
            End Select
        Case WM_KEYUP
            If frmMain.TabsAsSpaces And wp = 8 Then WasProcessed = True
            If frmMain.TabsAsSpaces And wp = 9 Then WasProcessed = True
        Case WM_CHAR
            If frmMain.TabsAsSpaces And wp = 8 Then WasProcessed = True
            If frmMain.TabsAsSpaces And wp = 9 Then WasProcessed = True
        Case WM_DEADCHAR
            If frmMain.TabsAsSpaces And wp = 8 Then WasProcessed = True
            If frmMain.TabsAsSpaces And wp = 9 Then WasProcessed = True
   End Select
    If Not WasProcessed Then
        'Default processing...
        CallWindowProc OldProcAddy, SubClasshWnd, Msg, wp, lp
        WindowProc = DefWindowProc(SubClasshWnd, Msg, wp, lp)
    End If