as Paul said is complicated to do more in this case , but if the client add order that mean is shold recieve invoice/proforma invoice for what he orderer , and maybe he ill pay it later . the thing that should be controlled here is :
1 - the order should never be placed or activated without invoice/proforma invoice should be paid . thais is already done and exist .
2 - the not paid orders shouldbe removed after X days if not paid . that i think is not yet implemented and we have here two scenario .
a - if the proforma invoice is enabled , the order should be canceled with the proforma .
b - if no proforma invoice enabled , the invoice shouldnot deleted , but it should be voided , and the order sholdbe deleted .
the best option that can be added is to make a condition , if the client has more than X orders be placed and not paid in the past don'taccept more order from itand convertthe account status to "inactive" .