It may be the case that while you’re happy to allow a user or process to have public key authentication access to your server via the ~/.ssh/authorized_keys file, you don’t necessarily want to give them a full shell, or you may want to restrict them from doing things like SSH port forwarding or X11 forwarding.

One method that’s supposed to prevent users from accessing a shell is by defining their shell in /etc/passwd as /bin/false, which does indeed prevent them from logging in with the usual ssh or ssh command syntax. This isn’t a good approach because it still allows port forwarding and other SSH-enabled services.

If you want to restrict the use of logins with a public key, you can prepend option pairs to its line in the authorized_keys file. Some of the most useful options here include:

• from="<hostname/ip>" — Prepending from="*.example.com" to the key line would only allow public-key authenticated login if the connection was coming from some host with a reverse DNS of example.com. You can also put IP addresses in here. This is particularly useful for setting up automated processes through keys with null passphrases.
• command="<command>" — Means that once authenticated, the command specified is run, and the connection is closed. Again, this is useful in automated setups for running only a certain script on successful authentication, and nothing else.
• no-agent-forwarding — Prevents the key user from forwarding authentication requests to an SSH agent on their client, using the -A or ForwardAgent option to ssh.
• no-port-forwarding — Prevents the key user from forwarding ports using -L and -R.
• no-X11-forwarding — Prevents the key user from forwarding X11 processes.
• no-pty — Prevents the key user from being allocated a tty device at all.

So, for example, a public key that is only used to run a script called runscript on the server by the client runscript@client.example:

command="runscript",client="client.example",no-pty,no-agent-forwarding,no-port-forwarding ssh-rsa AAAAB2....19Q runscript@client.example

A public key for a user whom you were happy to allow to log in from anywhere with a full shell, but did not want to allow agent, port, or X11 forwarding:

no-agent-forwarding,no-port-forwarding,no-X11-forwarding ssh-rsa AAAAD3....19Q user@client.example

Use of these options goes a long way to making your public key authentication setup harder to exploit, and is very consistent with the principle of least privilege. To see a complete list of the options available, check out the man page for sshd.