Was your contract written? Did it contain a restrictive covenant prohibiting them from competing with you within a reasonable marketing area, reasonable scope of activity to be curtailed for a reasonable period of time?  Does your state enforce restrictive covenants?

Did your contract define the identities of your clients as trade secrets  disclosure and competition with which would result in ireparable harm to you? Does your state have a trade secrets statute that it enforces. All of the above are possibilities, but you need to consult a local attorney about it.