Ignore them.. Or be immature yourself.. That can be real fun:p
The company I worked for, we had a woman who was so immature it was actually unreal. We got so fed up with it that for her birthday we all pitched in and brought her diapers, a pacifier and a home made card that said to GROW UP! inside.
No it's not nice and really wasn't the right the right way to go about it. But it worked, she actually seek-ed help in her immaturity. She did get better in time, we could actually deal with her more.
A good backhand strike to the crotch, (with the knuckles hitting the balls or vangita) followed by an up elbow strike to the face by opposite arm in fluent motion, then hook to smash, knee strike, hip throw, then whilst the subject is on the floor in pain, claw the entity mouth and pull the silly cretan along the office floor for utter humiliation.
I know this might sound a bit immature as well, but... ignore them.
Or at least not put your energy into them until they make the connection that their immature behaviour leads to your disinterest and that if they want you interested again, they should mature at least a bit.
Ask them:Did you ever think of growing up anytime soon? Or just say GROW UP!!!!!!!!!!!!!!!
at least try to discipline one and if it doesn't work then you should give him/her a warning
two middle fingers, straight up..........no waiting.