The system had allowed customers to write abuse into the reference field when making a bank transfer.
Westpac said it had found 115 cases this year, and while some were clearly abusive, others were joke messages.
The bank's manager of customer vulnerability, Louisa Brock, said the bank had contacted customers and warned them to stop or they could have their accounts closed.
No-one has had their bank account closed for this reason as yet.
However, Westpac was now changing its terms and conditions to make sure it clear that abusive messages were not acceptable, and it would drop customers who used offensive language.