You can't make business decision on fear of public media retaliation. You holding self hostage if you start second guess decisions just in case of this.

Terminate. Be professional. Do it in person but back up in writing with detail of final day of notice, fees owed and when due. And wait.

If something happen, deal with it then. Depending on what said, send a cease and desist letter recognising that they likely acted in frustration but state clearly that the offensive posts are not factual and therefore need be removed or you will take legal advice and potentially be seeking damages for the business impact their comments have.

If they post review on your site, don't ignore it. Respond one time, professionally, and state that you understand they are frustrated with not being a match, and wish them well going forward. If it continue, then one more comment directing them to your lawyer. Do not engage in back and forth and not give your side of story as that unprofessional.

Some people will go to social media regardless of what the real issue is and how you handle it. But what you must not do, is stop making business decision for fear this could be outcome. Cross bridge when get there.