Every now and then, someone doesn't get the act together and pay what they owe you. It's up to you when you decide to cease delivery, but you need a non payment policy in place, on principle.
You're within your rights, in most circumstances, to refuse to supply a person who doesn't pay. Unless payment is disputed, the money is payable.
Always check for legal issues in matters where payment is due.
Notification of your cessation of services can be coupled with a demand for payment, or done separately.
The example below is both a letter of demand and a notification. As you can see the demand for payment is specifically defined in the letter notifying of cessation of supply.
Notice of cessation of deliveries pending payment
You are hereby notified that no further orders will be supplied pending payment of your outstanding account.
Orders outstanding:Order 1: $237.50
Payment for these orders is demanded forthwith.
Please be advised that if payment for the above orders is not received within one calendar month of the date of this letter, action to recover these monies will commence without prior notice.