"Dear <customer>
Our auditors have reviewed the business and noted the ongoing 3% reduction despite the fact that the terms of the discount have not been met. Their advice is to make a claim for arrears plus interest. On foot of the advice, I am ceasing to offer a 3% reduction and would like to request a meeting with you to discuss the arrears issue in the hope that we can come to a mutually acceptable solution"
Framed like that I'd be fairly sure you will get your discount cancelled
Other alternative is to leave it as is, raise prices by 5% and then in a couple of months bring up the fact that the discount only applies for accounts paid within 30 days.