Removing default "ubuntu" user from Ubuntu Install on EC2
Solution 1:
Of course you can remove the ubuntu user, just make sure that the new user you created is able to sudo, etc..
All in all it doesn't really matter what your users are called. Services like cloudwatch also don't log into your instance to gain vitals. The instance is a virtual environment, what Amazon exposes is available to them from the host system.
Also, I don't see this user as a security hole -- by contrast, it was added to the Ubuntu AMIs because every other AMI allows you to login with root. That's more of a security issue. ;-)
Further more, the account is protected with a private key so I don't see an issue here unless your private key is compromised. In which case you should generate a new pair on the AWS console.
If you really want another login, you could try to rename it: usermod -l NEWNAME ubuntu