You can avoid the problem of defamation. All you have to do is create a list of criteria (including timely payment) for clients to get on the "A" list. If they do not conform...they are off the list. It is not a matter of defamation, but rather a failure to comply with the criteria required of the "A" list status.

I no longer  perform abstracts unless we are also given the closings. There is so much more money in the closings. I make it clear to every client that payment is due in 30 days. If they will not agree to it, we will not accept the order. We ocasionally make exceptions for a 45 day turn around in payment if the money is good. If the client agrees to the appropriate payment period, but fails to follow through with it...we no longer accept their orders subsequently.