Open the 
Sales & Receivables
Setup window.
On the General tab, in the Credit Warnings field, select No Warning (or Credit Limit if you want to keep this type of warning).
You can decide not to have warnings when a customer has an overdue balance, that is, an amount whose due date has passed.
Open the Sales & Receivables
Setup window.
On the General tab, in the Credit Warnings field, select No Warning (or Credit Limit if you want to keep this type of warning).
Eliminating Credit Limit Warnings