Old Nameserver IP's

I have resellers that have left my hosting and used to use their own nameservers using IP's I provided. Other than asking them to remove their nameservers from their domain registrars records, is their any way of cancelling their old nameservers?

The problem is the IP's can only be used once for use as a nameserver and it is preventing my new clients from registering their nameservers with the same IP.

 

 

 

 

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