How do I modify /user to a custom URL?
See the Rename Admin Paths module. The purpose of this module is to secure drupal backend by overriding admin path. You can change user/login to something like member/login.
I would also suggest adding a redirect in your htaccess to the standard user/login user/register. This way Drupal doesn't also have to serve up the 404 when a bot tries to access those pages. Huge performance savings.

EDIT: Thinking about this further, you don't need an extra module at all. Just add an alias at /admin/config/search/path/add for user/login, user/register etc, then just add redirects in your htaccess for the old aliases.