One thing you can do to prevent a user from logging in is to disable shell access for them (this will not affect their ability to log in via a POP server, however).
However it might be easier to avoid creating a user at all: Use sendmail aliases to redirect, and/or procmail to deposit mail to a specific mbox. Some POP servers can be configured to serve up eMail from mbox's to mail-only users (i.e. these are virtual mail-users and do not require an actual user account on the box).
Also, i think postfix handles this more easily than sendmail (although i haven't actually used postfix so i don't know for sure). I do plan to migrate to postfix soon, however, as it looks to me to be more secure than sendmail.