Rental of trailer - refuse to pay

They rented a trailer from our company on the 29th of March 2015, they advised they will be using the trailer for a long period and we agreed to reduce the daily cost. They had the trailer in their possession for 26 days. They still have an amount outstanding. We have sent them emails, phoned them, sent them sms's and they are just ignoring us. This is not on. I wouldn't do business with them, so keep an eye out for them.