restricting ssh to authorized users

[Robert Storey]

I've been wondering what is the best way to prevent certain users from
being able to login with ssh, even though I want to allow them ftp access?

The opposite is easy to accomplish - if I put somebody's name in file
/etc/ftpusers, that person cannot login with ftp, but they could still
login with ssh. But I don't see a file /etc/sshusers, and I'm wondering if
there is some equivalent.

TIA,
Robert