Whenever an app freezes on me (which is rare) I just hold down the power (end call button) for about 5 seconds until I get the shutting down screen. I have yet to see that not work. This way you don't need to go through the entire boot process.
when my BBM Group pics freezes up, i can use the app switcher to navigate away from it. however, it still remains frozen and messed up. as far as ive seen, rebooting is the only real way to fix a frozen app.
the device should kill it itself after a while... i know sometimes when i have an app (i do alot of beta testing so it happens) that freezes and i get out of it via the task manager etc... the device after a while sees its not responding and will kill it by itself... you'll see a dialogue box telling you that it killed it