There are a few simple solutions to solving the "I am an actor, I don't need to put away after myself" syndrome
First, you inform them that putting the props away is their responsibility. If they don't put them away, then they won't be there next show.
Still collect them if they don't put them away, but don't put it with the rest of the props. Next show, they will ask where their prop is, say "Did you put it away last performance?" They will either lie (if they do, call them on it) or they will tell the truth, and you should pull it out of your bag/kit/whatever and say something along the lines of "I expect that you will put it away tonight?". If they fail to put it away again, send them to go and find it (even though you have it). Then when they cannot find it (10 or so minutes later) say "Your lucky that X member of the public found this and handed it in then.". Third time, they don't get the prop, and you say you will find a new one before the next show. I doubt it will even get that far.
Another method: At the start of the run, tell everyone that they are expected to remain behind for half an hour to assist with the pack down. Then draw up a roster of duties. Distribute it. If they say "I have to go meet friends", tell them that you were upfront about the time requirements, and that their friends can wait until they have finished their rostered duties. If there was an important meeting, you should have been informed well before the pack down.
And if all else fails, sit them all down and lay on the guilt.
Emotional blackmail works.