It depends on the compilation target: I'm going to assume you're compiling to CPP (Windows EXE). In which case you should just be able to use the following:
import flash.system.System; // Or nme.system.System if you're using NME
...
// in your FlxButton callback:
System.exit(0);
I can't test right now so I don't know what effect this would have in Flash (i.e. you may have to wrap it in a conditional compilation flag for cpp), but I do know that it won't work for iOS.