That menu takes a little effort to get at. But you can handle CurrentCellControlGotFocus and explicitly set it there. If you want the menu to be your menu, then just it as below. If you want the menu to be empty, create an empty menu, and set c.ContextMenu to be that empty menu.
private void gridControl1_CurrentCellControlGotFocus(object sender, ControlEventArgs e)
{
foreach(Control c in this.gridControl1.Controls)
{
if(c is GridMaskedEditBox)
{
c.ContextMenu = myContextMenu; }
}
}