How do I force SSH to use password instead of key?

You try using correct parameters for disabling authentication over keys. Maybe server reject password authentication? Check server ssh configuration.

Permission denied (publickey)

The "(publickey)" string in that error message is the list of authentication methods accepted by the remote server. In this case the remote server only accepts public key authentication. You can change your client parameters all you like, but it won't alter the fact that the server will only accept public key authentication.

To log in with a password, you'd have to start by reconfiguring the remote server to accept password authentication.

Turns out the solution was to set PasswordAuthentication yes in /etc/ssh/sshd_config.

Thanks to the several people who pointed me in the right direction.