Hi
not sure if this will help but I got so fed up with delivery men leaving our gate wide open despite 3 notices ie dogs running free , please shut the gate in 2 sizes etc , plus we put a large bell on the gate , and a push button bell that rings very loudly in the house ! - so in the end bought one of those number padlocks things so gate only opens a tiny bit before they get message cant get in ! plus dogs can't get out !!
so far its working - this prob not work on a front door am guessing . but maybe like mentioned by someone else a huge shut the door sign would be good !