Revoke the Access Of EventHandller of Datagridview

Question

I have Bound Datagridview and and trying to access of editing control as below:

private void newdgv_EditingControlShowing(object sender, DataGridViewEditingControlShowingEventArgs e)
    {
        if (newdgv.CurrentCell.ColumnIndex ==0) 
        {
            if (e.Control is TextBox) 
            {
                TextBox tbox = e.Control as TextBox;
                tbox.KeyDown += new KeyEventHandler(tbox_KeyDown);
                tbox.TextChanged += new EventHandler(tbox_TextChanged);
            }
        }
    }

    private void tbox_TextChanged(object sender, EventArgs e)
    {
        TextBox tbox = (TextBox)sender;
        if (tbox.Text.Length  > 2)
        {
            listBox2.Visible = true;
            button2.Visible = true;
            int inddex = listBox2.FindString(tbox.Text);

            if (inddex != -1)
            {
                listBox2.SetSelected(inddex, true);
                listBox2.Focus();
            }

        }
        tbox.TextChanged -= new EventHandler(tbox_TextChanged);              
    }

Above at tbox_Texchanged EventHandller I am trying to Revoke the access of the same due to avoid of repeating same task on other columns's control. But it won't allow me above task. I can't see Listbox2 on passing the text limit of control.

Now look at the same example in other way:

private void newdgv_EditingControlShowing(object sender,  DataGridViewEditingControlShowingEventArgs e)
    {
        if (newdgv.CurrentCell.ColumnIndex ==0) 
        {
            if (e.Control is TextBox) 
            {
                TextBox tbox = e.Control as TextBox;
                tbox.KeyDown += new KeyEventHandler(tbox_KeyDown);
                tbox.TextChanged -= new EventHandler(tbox_TextChanged);              
                tbox.TextChanged += new EventHandler(tbox_TextChanged);
            }
        }
    }

    private void tbox_TextChanged(object sender, EventArgs e)
    {
        TextBox tbox = (TextBox)sender;
        if (tbox.Text.Length  > 2)
        {
            listBox2.Visible = true;
            button2.Visible = true;
            int inddex = listBox2.FindString(tbox.Text);

            if (inddex != -1)
            {
                listBox2.SetSelected(inddex, true);
                listBox2.Focus();
            }

        }

}

Now see here it is allowed to see listbox2 on Column "0" and "1" Controls by passing of text limit. But I just want to fire the even on only column="0"'s Controls Not on Columns="1"'s Controls.

I Confused Bewteen Revoke the Access Of EventHandller.

How to Do?.