I would consider finding an attorney particular to your circumstance, business and/or industry. I would then contemplate the choice with the idea you may need to use the attorney for next steps, or even to defend against a counterclaim.
Most will tell you a cease and desist demand and defamation claim are easier said, much harder done. They can be effective in intimidating the other party into cutting out the behavior... and are often necessary to take the next step in making an actual defamation claim.
But, ever getting an injunction on the other party or getting any damages from a claim can be very difficult. But I am not an attorney, I just have to pay them from time to time. And we had this issue come up with a competitor talking smack about my company to other companies and the agencies we worked for. The letter followed by a nice in person meeting letting them know the seriousness ended the behavior. But we stood no chance of ever recovering any damages from their defamation type activity.