How to disable certain string inside text box?

What I done as code below show that all the data inside text-box was disable.

Private Sub chkSTC_CheckedChanged(sender As Object, e As System.EventArgs) Handles chkSTC.CheckedChanged
        Select Case chkSTC.Checked
            Case True
                txtCargoDescription.Enabled = False

            Case False
                txtCargoDescription.Enabled = True

        End Select
    End Sub

End Class

How to disable only certain string which is "1 X 40 HQ STC" from text-box. What should I add? Please refer this image

Thank you in advanced!