If you have proof you can firstly be ready for a legal battle, for once you take action they may react. What you can do is to give a police compalint against them with proof. Firslty you will have to issue them notices for absconding without prior notice and for not handing over the company property in their posession. You will have to make a list of the items in their custody when they left.
NExt you will have to issue a lawyers notice seeking specific relief of return of company property and settling all issues apart from compensation for this abrupt absconding from service.
Last but not least you could give a police complaint against them assuming theft of company property as they have left abruptly and you have found the property missing (give list).
Finally if they are trying to compete with you using the information, you could also contemplate putting an advertisement cautioning all about them.
Going into details sitting with an Advocate would be better as most of the advice here would be suggestive only. Without knowing the inner details it would be difficult to judge what action you should specificallly take.
20th November 2009 From India , Bangalore