Solved by my own.
Here's the Code:

Private Sub Username_KeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles Username.KeyPress
Select Case e.KeyChar
Case Chr(65) To Chr(90), Chr(48) To Chr(57), Chr(8) 'allows a-z and 0-9 plus backspace
Case Chr(97) To Chr(122), Chr(8) 'allows a-z plus backspace
Case Else 'everything else than above
e.KeyChar = Chr(0) 'wont be accepted
End Select
End Sub
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