In other words, customer is asking a court to rule whether or not IP space should be portable, when an industry-supported organization (ARIN) has made policy that the space is in fact not portable. It can be further argued that the court could impose a TRO that would potentially negatively affect the operation of my network.
A court will likely decide this based upon the terms of your contract and what the court thinks is fair. They will likely give very little consideration to common practice or ARIN's rules.
Another VERY important issue to bring up: If customer is granted the legal right to continue to use IP space that is registered to NAC by ARIN, NAC runs into the very serious problem of being liable for all of the Spam that could be generated by the customer and all of the RBLs that the carrier may be added to [that of course will effect all of NAC's customers] with no ability to revoke the IP space to protect itself. This has to potential to effect the NAC network in a catastrophic manner.
You'll just have to explain to people that the traffic didn't originate on, terminate on, or transit your network and therefore there is no justification for holding you responsible. When arguing against the TRO in court, make sure to point out that this TRO would make you responsible for behavior over which you have no control. However, the court will likely find that failure to grant the TRO puts a greater hardship on your soon-to-be-former customer than granting the TRO puts on you. Courts do not look well on artificial attempts to penalize a customer for changing providers. Lock in is considered anti-competitive. They will likely see your revocation of the IP addresses (or failure to offer them separately for a reasonable fee) as a case of lock in. Standard industry practice, AFAIK, is to allow customers to keep their IP addresses for a reasonable amount of time unless you have always had a policy of not allowing customers to advertise any of your IP space through any other providers ever. IANAL, seek competent legal advice from a lawyer with experience in this area. I'm sure you can work out some sort of compromise where you let them keep using their IP space for a reasonable period of time (3 months? 6 months?) and they renumber in that time. I'm fairly sure they don't expect to keep your IPs forever and I'm fairly sure you don't need them back immediately. DS