Go to file /etc/ssh/sshd_config

Find param `ChallengeResponseAuthentication` and set to `no`.

```bash
ChallengeResponseAuthentication no
```

Find param `PasswordAuthentication` and set to `no`

```bash
PasswordAuthentication no
```

Find param `UsePAM` and set to `no`
```bash
UsePAM no
```

It may also be a good idea to disable root login
```bash
PermitRootLogin no
```


Reload ssh server with `/etc/init.d/ssh reload`