In such a case you still have the option to make the behavior physically impossible, by placing a physical barrier around the dangerous object; no different from a fire-guard in front of an open fire, or a childproof gate at the bottom of a flight of stairs. In my own experience, my kids got the message about the stove by being told sternly not to touch it and why it was dangerous, calmly but firmly and seriously. Other stuff took repeating, but never more than 3 times (kids have moments when they're just not taking information in, and we're constantly telling them what to do, so factor that in, you need to repeat the really important stuff)