What happens in every horror movie when the group splits up? People die.
What happened in Jurassic park when the characters went off in different directions? Death
At this point most of my routine players have learned that you either work as a group or I systematically start selecting errant players and either eliminating them or have backlashes so severe that they also effect the people working as team. But I try to never do it explicitly out of spite. If I do it, it's going to be setting appropriate and effect the overall narrative no matter what.
But I've found that the best way to get people to work together and have them come up with some exceedingly clever solutions is to put my proverbial foot down and demand it with full on "adapt or die" situations. If someone's about to do something REALLY dumb I'll contact them privately and strongly advise against that particular course of action. Of course I don't give a reason WHY, that would disrupt the narrative and provide inside knowledge of future events and possible repercussions. But in general I've found the best method to simply be to start expending their resources, maiming or mauling characters, or flat-out killing them off.
And some people have gotten upset over it. I'm not. And most of my players haven't. Because if they want to write fan-fiction about how awesome their character is then they can go do it. But if they want to participate in a shared narrative and tackle setting specific problems then they should be willing to play ball.