Teach them that touching the screen results in harsh punishment. You may even want to refer to it as the "no-no" (or some other stupid term that clearly means "don't touch"). Eventually they'll get the idea.
Keep small, throwable items away from the desk (i.e. no pens or pencils in sight). These things seem to turn into projectiles when kids (and some adults) get mad.
Keep small, throwable items away from the desk (i.e. no pens or pencils in sight). These things seem to turn into projectiles when kids (and some adults) get mad.