Better Business Bureau, Yelp and other online commercial websites that allow you to post reviews of their firm, are great places to mention their lack of service and the quality that it conveys to potential future clients.

Be artful with words, as a kind word can be damning:  i.e.  "We appreciate the lack of communication as it has kept our mail clear of payments and our email bin devoid of replies to our many polite inquiries about their unpaid balance."

Find them on FaceBook, LinkedIn, MySpace, Tumblr, Blogster, LiveJournal etc... by using Spokeo, and posting to their business accounts on each.

Complain to their local Chamber of Commerce too.

There are even complaint boards and websites that  you can Google that let you bitch them out online.   

 This all hits them in the pocket book with future potential clients who will think twice before using them.

 Remember, that a business that does not pay it's suppliers can't rely on getting quality goods for it's clients.  This is a simple and obvious function of the free market, and it's good to remind people of  this.