POOR PAYMENTOF ACCOUNTS

I have been a supplier of Builit Mercury for many years with no payment problems ever. Since the company was taken over a couple of months ago I have been having endless problems with them paying the account. After a number of emails, phone calls and empty promises from the account department to date I am still trying to get payment from them for goods supp****. It has now reached the stage of me refusing to supply them any products unless it is paid for in full on order. The sad part is that they have been paid by their client for the product that I supply. Why should I have to beg them to pay the account.