User set passwords

There is a field in General Options - Registration/Profile Fields & Options which enables you to have users choose their own passwords.

@kts915’s suggestion is solid if you want conditional removal of CSS, but if you want to just remove s2member CSS files under all circumstances, then take a look at the last comment in the following post:
https://www.s2member.com/forums/topic/remove-css-and-optimize-js/