Se tu fossi in visual basic faresti così:

codice:
Private Sub Text1_Change()
c = Right(Text1.Text, 1)
If c = "" Then Exit Sub
If IsNumeric(c) = False Or c = "," Then
    Text1.Text = Left(Text1.Text, Len(Text1.Text) - 1)
    Text1.SelStart = Len(Text1.Text)
    Text1.SelLength = 0
ElseIf Text1.Text = "" Then
Exit Sub
End If
End Sub
traduci in Vb.Net

ciao