I've got one of those brake disc alarms, brand name ZENA, for my front brake disc. I put a regular disc lock on the rear brake disc. For a padlock I use one of those round padlocks. Anything can be stolen, I'm just trying to make it hard enough for the bad guys to go steal someone else's bike.
http://m-jroadstore.com/ click on products and then go to security. The Immobiliser was used by most Harley dealers untill Harley came out with their own system from the factory. The Immobiliser is a decent system for the price, it's a plug and play set up, no cutting wires, can by easily installed or removed, has a pager to let you know if the alarm has been activated if you are out of range of the siren. I put one on my wifes sportster, I have one on my bike also. By the way, this guy that sells them is the one who invented this unit, he has the patent, He use to work for a company that use to market them, never mind it's a long story, He will be glad to tell you all about it if you call him, he is very helpful and stands behind his product.