I assume that he did this while employed, and at a time when he was authorized to modify your website (for example when this was actually his job). There have been cases where companies tried to use anti-hacking laws in cases like this, but this seems to not work anymore. There was the case of a woman whose job it was to take money for lottery tickets and print them out using a computer; she used the computer to print about 1,000 tickets a week for herself! This turned out to be theft, but not unauthorized use of a computer, because she was actually authorized. Same in your case.
You can sue for damages - damage that he actually caused to your company, possible profits that he made through his actions, possibly salary that he was paid while working for himself, not the company. This may all be quite difficult. Can you prove any financial damage? Or damage of your reputation? Sending people to his app in the app store likely doesn't damage your reputation. And he might have done all the work after office hours so he never got paid for it.
PS. Someone posted this as an answer: "At Wallin & Klarich, we have over 30 years of experience in criminal defense and family law matters. Our criminal defense attorneys and family law lawyers are committed to providing our clients with the highest quality legal representation." Which clearly raises the question, if an employee or ex-employee damages the reputation of a law firm by spamming on law.stackexchange, what legal consequences could that employee face?