EC2 Public IP Instance Deleted but still responding to pings

A public IP address is assigned to your instance from Amazon's pool of public IPv4 addresses, and is not associated with your AWS account. When a public IP address is disassociated from your instance, it is released back into the public IPv4 address pool. So it is possible that it is used by someone else already. If you want to use Static IP allocated to your account then you would use Elastic IP.