Well, you can try pulling the battery, but if it is like most, and still under warranty, then you aren't going to want to open it up (which would void the warranty).
In this case, you will have to let it run out of power to actually get it to turn off. Then once it is off, plug it in to charge for a short while. Only a short while because you want to be able to test it to see if it will work fine and should it freeze again, then you will have less time to wait until you can get it to be off again.
Also, take note of what you are doing and have running when it freezes. Any new apps you installed could be the cause. Also, leaving many apps open at once can cause this as well. So remember to actually close apps when not actively in use...