Like Nathrao and Saswata Banerjee mentioned, you should have included non-solicitation clauses vis-a-vis your employees, vendors, and I add, as well as your clients.
Handling the Issue
To address this issue, collect evidence to support criminal charges against your ex-employee. Check his official emails. I hope you have access to them through the servers. Generally, employees with such criminal intent forward emails to others through their official mail IDs. That would be proof enough to demonstrate criminal intent and the extent of damage (for all you know, some of your clients may also be on their list to poach later).
Points for the Future
In business, never make decisions 'through the heart.' Use your head, especially when those you know are involved.
I recollect a small story here: "When someone who got betrayed asked the betrayer of the trust reposed on how he could do it when he was trusted so much, the answer was: I could betray your trust only because you trusted me; if you didn't trust me, how could I manage to betray you at all?" The moral of the story being: Repose your trust after careful choices.
And like other members suggested, consult a lawyer ASAP to see if the damage is just what you know now or if there is something more coming later, basically damage control.
All the best.
Regards,
TS