I have probably had a rant on this before but every time it happens it winds me up.
We expect people to put the right reference on BACS payments to us. Its not unusual. Its why BACS has an 18 character reference in the first place - for the benefit of the beneficiary. If you pay water, gas, electric, telephone, rates, council tax, VAT, corporation tax, PAYE contributions to HMRC, heck - almost anything, you have to put the right reference. We're not unusual.
Occasionally a new customer will put something daft, we'll eventually work out who it is (sometimes by waiting for them to complain that we are chasing a bill that has been paid) and explain that they have to put the right reference. That usually sorts it. Policy is to actually send at least two real letters by post explaining this... Only if they persist do we charge an admin fee of £5+VAT for manually handling the payment (cheap price for my time I think).
When we charge £5 admin fee for this we try to include it on the normal invoice, but this does not always work as planned especially on quarterly bnilled customers, so sometimes our system raises an invoice for just £5+VAT which states "Admin fee (reduced): manual processing of incorrect BACS/CHAPS/FP payment. PUT THE CORRECT REFERENCE ON YOUR PAYMENTS IN FUTURE TO AVOID THIS CHARGE. See http://aaisp.net.uk/bacs.html for more details."
What really takes the biscuit is when someone pays this £5.75 invoice, by BACS, without the right reference on the BACS payment, so that I have to manually allocate the payment!
Are they totally mad?
I used to have a special letter I sent in such cases, but basically if people have ignored the letters already they ignore that letter too, so there is pretty much no point in doing anything other than just charging £5.75 again for handling the £5.75 payment they just made.