From http://www.vbforums.com/showthread.php?570438-Restrict-TextBox-to-only-certain-characters-numeric-or-symbolic Disallow characters in textbox, could apply to other field types.

Disallow_chars.snippets

Public Class MainForm

    Dim charactersAllowed As String = " abcdefghijklmnopqrstuvwxyz1234567890"

    Private Sub TextBox1_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox1.TextChanged
        Dim theText As String = TextBox1.Text
        Dim Letter As String
        Dim SelectionIndex As Integer = TextBox1.SelectionStart
        Dim Change As Integer

        For x As Integer = 0 To TextBox1.Text.Length - 1
            Letter = TextBox1.Text.Substring(x, 1)
            If charactersAllowed.Contains(Letter) = False Then
                theText = theText.Replace(Letter, String.Empty)
                Change = 1
            End If
        Next

        TextBox1.Text = theText
        TextBox1.Select(SelectionIndex - Change, 0)
    End Sub

End Class