This assumes you have already configured the aws command line tool. If not, check out my article on Getting Started with the AWS CLI.

The aws ec2 describe-key-pairs command can then be used to list the EC2 key pairs that have been created.

~]$ aws ec2 describe-key-pairs
{
    "KeyPairs": [
        {
            "KeyPairId": "key-0551234a925abcd7a",
            "KeyFingerprint": "86:0e:bb:db:6b:78:7c:67:0e:b1:91:38:0a:ce:95:71:4d:2b:72:2e",
            "KeyName": "default",
            "KeyType": "rsa",
            "Tags": []
        },
        {
            "KeyPairId": "key-04582539453c591c1",
            "KeyFingerprint": "b6:eb:8a:a0:5e:4e:a6:0b:66:3c:3d:93:6d:42:63:d7:f5:32:23:71",
            "KeyName": "my-key",
            "KeyType": "rsa",
            "Tags": []
        }
    ]
}

The aws ec2 delete-key-pair command can be used to delete a key pair using the name of the key pair.

aws ec2 delete-key-pair --key-name my-key

Or using the ID of the key pair.

aws ec2 delete-key-pair --key-pair-id key-04582539453c591c1