How can I create a non-login user?

You can use the following command:

useradd -r subversion

For more info, check manual pages with this command:

man useradd

You will find in this documentation the following flag that can be used for your purpose.

-r, --system                  create a system account

The -r flag will create a system user - one which does not have a password, a home dir and is unable to login.

You can use the -M switch (make sure it's a capital) to ensure no home directory will be created:

useradd -M subversion

then lock the account to prevent logging in:

usermod -L subversion

Another solution to create a system user, using adduser :

adduser --system --no-create-home --group yourusername

You can remove --group if you don't need group yourusername, and --no-create-home if you do need a home for this user.

As mentionned by py4on in comments, on some systems one may need to use the --disabled-login option in order to, well, disable login for this user. It seems to be the default behaviour under Debian, though.

Beware that the numeric ID of the user will be of a system account. You can fix the uid using the --uid option, though.

Finally, note that on some systems (e.g. Fedora) adduser is a symlink to useradd, in which case this answer is not valid.