I would give bitter apple spray a try, or some other type of spray meant for the same purpose. Make sure that when you spray it on the bars, you do it away from the rest of the cage so only the bars of the cage taste like bitter apple, and it doesn't get onto the floor of the cage, in food, water, etc... Typically all pet shops, local or corporate, will hold these sprays and they should be easy to find.
If you don't want to spend the money, some people online have claimed to use vinegar or lemon juice, but I'd do a bit of research on my own to make sure those are plausible solutions.
As James Jenkins mentioned in the comments above, look at his answer here before using additives on the cage.