The shell setting in /etc/passwd determines whether a Linux system user may log in via the shell or over SSH. If you don't want a certain user to be able to log in, set the shell to /bin/false or /sbin/nologin.
For the user "otheruser" on Debian and Ubuntu Linux, here's an example:
usermod -s /bin/false otheruser
For Redhat, Fedora or CentOS use /sbin/nologin:
usermod -s /sbin/nologin otheruser
Warning: Do not set the shell for the root user to /bin/false or /sbin/nologin!