Disabling mouse right click

Simply set the textbox property "ShortcutsEnabled" to false and you got it.
This way neither popup nor Ctrl&V will be accepted as textinput.
 
Regards.

Please refer:
Disable Copy/Paste in a Textbox in Your WinForms Application[^]
Disable copy-paste in textbox [^]
Disable Copy & Paste in a textbox[^]

use this code
 

private void Form1_MouseClick(object sender, MouseEventArgs e)
{
    if (e.Button.ToString().ToLower()=="right")
    {
    SendKeys.Send("{Esc}");
    }
}