Open the 
Sales & Receivables
Setup window.
On the General tab, in the Credit Warnings field, select No Warning (or Overdue Balance if you want to keep this type of warning).
You can choose not to have warnings telling you that the customer's credit limit has been exceeded.
Open the Sales & Receivables
Setup window.
On the General tab, in the Credit Warnings field, select No Warning (or Overdue Balance if you want to keep this type of warning).
Eliminating Overdue Balance Warnings