How do we keep 2 yr old from opening screen door?

I am one of the first to endorse and use the slap on the hand and the sharp "NO!"
However, I have enough experience with children to know that as well disciplined and as well behaved a child can be, there is always the time that they will still be testing the limits. I don't want to take the possible chance of a precious child falling out of my fifth wheel, down 3 metal steps and onto the pavement. It could cause serious injury. I appreciate the helpful advice that many gave!