There was a provider in town that I wanted to see recently and she was very high end and used an agency to screen / set up.  So I exchanged a few emails on Tuesday for a meeting later in the week.  
Some things came up for work and Tuesday night and I could no longer keep the Friday appointment.  Now the agency preso. I was working with is threatening to call my work and "ruin my name", if I didn't show up and pay the full fee regardless.  They don't have a website so there is nothing I have ever seen for them regarding cancellation fees or obligations.  I know it sucks on their end but I tried my best to give ample notice and even work out another day.

Many agencies do have such a policy so if you failed to read you should
adhere to their policy when you set up appt. if there is indeed one.

Its not worth your time-  i think theyre bark is bigger than their bite.  

Assuming you have perfectly characterized the exchanges, you need to let the board know the provider's name.  The ruining reputation thing is a two way street.  If she demands a significant donation for her time, none of us want any "drama".  Hopefully, said provider will realize this behavior is not good for business and will intervene.  Good luck.

It seems to be a common threat from crazy people who usually don't do anything, but some people are so screwed up that they may just speak with your work to be dicks. A cease and desist order basically tells them that if they slander your name to anyone you're associated with or contact you again then they will have to show up in court, which obviously they wouldn't do. Maybe ask your lawyer about a c&d. Do you mind PMing me the name of the agency? I'm sorry that happened, it's so petty  

There is no reputable agency nor indy that will risk their own well being    FACT

they get 400-500$ a night hotel in back bay and if at least one customer cancels it might make their trip to Boston unprofitable...this why they are very aggressive with cancellation fees and measures...

Thanks so much for the support, guidance and input.  

Tell them you gave plenty of notice, and to stop bothering you. Then ignore/block them from any other communication. They will likely just go away, as they are trying to shake you down.

both venting their anger at you and looking to make whatever they can ($$) from the situation.  

Please refer to the post below .. Never ever ever ever give a agency your info..  

Expose them

If that is the case simply respond that any further attempt on their part to extort money from you will be met with a criminal complaint being filed on them for attempted black mail and extortion.