Here's what I did on a Ubuntu EC2:
A) Login as root using the keypairs
B) Setup the necessary users and their passwords with
$ sudo adduser USERNAME
$ sudo passwd USERNAME
C) Edit /etc/ssh/sshd_config
setting
For a valid user to login with no key
PasswordAuthentication yes
Also want root to login also with no key
PermitRootLogin yes
D) Restart the ssh daemon with
$ sudo service ssh restart
Now you can login into your ec2 instance without key pairs.