[nicksgf]

Bitter spray worked really well for us (used it on the bottom of his potty pads so he'd stop playing with them!)
The can with pennies in it is a good idea - or a spray bottle with water in it.
I've heard that if you don't have one around you then just make a really loud noise - just enough to distract them. Or a really sharp NO.
Then as soon as they stop chewing and look at you, reward them for listening and distract them with something they are allowed to chew on.
A lot of people forget to reward them as soon as they look at you. If you don't reward them they learn your not going to give them anything, so they start to ignore you.