Disconnect the wires leading from the battery or the trigger/timer to the detonator. The battery will look like a battery; the timer is the part that's counting down. Be careful: This is the trickiest part of the bomb – and where it's most likely to be booby-trapped (the whole red wire/blue wire thing comes into play here). And for God's sake, don't fuss over the process in order to make the timer stop at something clever like 0-0-7.
I thought it would be worth linking to this story at Wired, just in case you happen to run across one. Sure, it might have been built by North Korea, in which case you're pretty safe, but the recyclers won't take it unless you break it up and sort it.