As all others had said, it is not clear what exactly you are looking for. If you want to restrict user from accessing other files, then CHMOD will work for you. But, if you want to lock the user from log-in at all, then use :
usermod -L <username>
and this will lock the user in /etc/shadow file. To unlock the user , use :
usermod -U <usermod>
Remember, that these commands can be executed by administrator only.
Apart from these, there are few more ways of restricting users from logging in , or log-in in a restricted mode through /etc/inittab file, but those are a bit tricky and risky as well.
Hope this will help you.