We have a customer that paid late and is now trying to hold us responsible for work delay because the equipment and supplies needed cannot be delivered on time for their target opening. They sent an official delay of work letter. We would like to defend against the delay claim (our contract clearly states payment must be made prior to purchasing any supplies or equipment and may cause delays in scheduling) they were also reminded of this 4 times between 5/11/2020 and 5/22/2020 when attempting to confirm scheduling. We would also like to terminate the relationship entirely. This customer has a substantial past due balance as well so the relationship just needs to end. How can we go about doing those things in a legal and professional way