Last year I was doing Addams Family and Lucas decided that, in a serious moment when he is supposed to be impressing Gomez, his pastimes included training squirrels in the park instead of helping orphans with their homework. Unfortunately this was not the only improv for this line, despite a talking to by both myself and the director. He also used Wednesdays braids (her hair was about 3 ft. long) as horse reins, "giddy-up"ing her on stage and decided my ASM was waiting too long to open the grand after intermission and took it upon himself to open it for her. When I tried talking to him about it, in as calm and stern a manner as I could, doing my best not to yell at him, he just put up a wall and wouldn't listen. In this case he was on the opposite end of the age scale, 19 and irreproachable. The only thing that got him back on track was telling him that his mentor was in the audience, after that he was a gem.
Is there a better way of handling people with that kind of attitude, any key phrases that will take their walls down?