Disable a particular key

First get a event of your text feild (like text box) in your project

double click on this event of your textbox or richtextbox

this event occurs when ever a key is pressed

now the code,
 private void richTextBox1_KeyDown(object sender, KeyEventArgs e)
        {
 


            /////////// do your work here
            if ( e.KeyCode == Keys.A)
            {
                e.Handled = true;
            }

       
        }





Enjoy the joy of Coding
God Bless You
Jai SiyaRam