Messed Up Name Servers.

Hi when I go to, Manage Name Server IP's it goes through all the IP's how ever when i try to remove ones that are from past customers " no longer with us" it gives me this error.

Sorry, the nameserver ip xx.xx.xxx.xxx cannot be removing since it is still registered with an ICANN register.

I was wondering if theres a way to manualy remove this as we need them for new customers. Also i was wondering if i have to enter in the name servers into cpanel or just inside of there Registars Domain Pannel. Because it wont let me specify which IP it uses it keeps ranodmly pulling IP;s when i do the assign name server IP.