At what point should I cut off slow paying account customers from additional credit purchases?
Probably depends on what kind of relationship you have with your client; how crucial they are for your overall revenue; what proportion of total revenue they make up and whether you want to retain them or not.
Personally, I ditched offering credit terms for clients many years ago. And when I did offer credit, terms were no longer than 45 days.
It's crucial you A. have a signed agreement/contract that clearly spells out your T&Cs, and; B. stick to them without fail.
You will want to review your credit process - I find that by addressing such issues up front ie strategy will save you a lot of energy with all the tactical stuff later on. There is no hard and fast rule when it come to business, in the end we are dealing with people, so need compassion use your judgement, if you feel you are getting taken advantaged off then respond accordingly. Notice how i said ''respond'' not ''react''.