Typically services set an internal "comfort" limit. When a client passes that limit, it is flagged internally for review. You can then risk-assess: look at who

Typically services set an internal "comfort" limit. When a client passes that limit, it is flagged internally for review. You can then risk-assess: look at who the customer is, what are they doing with it, do they have an incentive to pay a large bill (i.e. would it kill their service if you cut them off for not paying). You can then decide to let them continue, contact them for an interim payment, or suspend their service if the risk is too high. While you're risk-assessing them, they can continue to use the service (it's not automatically suspended) so the customer need not know what is happening. And of course, its usually wise to factor in a level of non/tardy payment into your cash flow projections, as a realistic cost of doing business.