Use euca-disassociate-address and euca-release-address to disassociate an IP address from an instance and to release the IP address to the global pool, respectively.
To release an IP address:
Enter the following command to disassociate an IP address from an instance:
euca-disassociate-address <IP_address>
Enter the following command to release an IP address:
euca-disassociate-address <IP_address>
The following example releases the IP address, 192.168.17.103
euca-release-address 192.168.17.103