using System;
using System.Windows.Forms;

public partial class Form1 : Form
{
    private class RichTextBoxNative : NativeWindow
    {
        public RichTextBoxNative()
        {
        }

        protected override void WndProc(ref Message m)
        {
            // WM_MOUSEFIRST, WM_MOUSELAST
            if (m.Msg >= 0x0200 && m.Msg <= 0x020D)
                m.Result = IntPtr.Zero;
            else
                base.WndProc(ref m);
        }
    }

    public Form1()
    {
        RichTextBox rtb = new RichTextBox();
        rtb.Dock = DockStyle.Fill;
        rtb.ReadOnly = true;
        rtb.Parent = this;

        rtbn = new RichTextBoxNative();
        rtbn.AssignHandle(rtb.Handle);
    }

    RichTextBoxNative rtbn;
}