A TextBox handles key operations in KeyPress event.
So, You can surpress Enter button in KeyPress event by changing the KeyPressEventArgs.Handled property to true when KeyChar property is 13.
this.textBox1.KeyPress += new KeyPressEventHandler(textBox1_KeyPress);
private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
{
      if (e.KeyChar == (char)13)
      {
            e.Handled = true;
       }
}
Today, I went to Mori Art Museum at Roppongi in Tokyo. I saw a 
exhibition of Odani Motohiko. His arts are a little grotesque, but exciting. Also, colored leaves were beautiful in the park near the museum.
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