Contact - Font color
If we cannot put a warning against the bad payers, surely it cannot be that hard to be able to change the customers business names into another color (Red) So when you search or create a new invoice, alarm bells will ring and hopefully avoid more loss

Hi Todd, have you seen credit limits? These will allow you to set a limit to how much spending a customer can make with your business. When they reach that limit you can also choose whether to restrict invoicing for that customer. This may help with the scenario you're noting here.
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Kathleen Caden commented
In our old system, before we moved to Xero, we could highlight special clients (they were a different colour). In our case, they were special because they paid their account off in instalments, instead of paying a lump sum. This made it very easy for us to separate them when we sent invoice reminders to the rest of our clients. How we have to do it in Xero now is clunky and manual. :)