You can add or remove that field from the Public Access New Member Registration Form and the Constituent Membership Join form using Configure Fields & Sections, which is found at Global Settings cog > Global Settings > Forms & Web Pages > Standard Forms > Configure Fields & Sections.
You might have a header on each page within your login portal that displays membership status and gives the option to renew.
If you'd like to turn this off, navigate to Global Settings cog > Global Settings > Memberships > Settings.
There, you'll find a switch to disable this header information.
Finally, you can remove the option to renew from the membership page within your constituent portal here: Global Settings cog > Global Settings > Constituent Login Portal > Dropdown Navigation Options.
From that page, click Edit next to the standard menu option "My Membership".
This takes you to the configuration page for that menu option where you can choose to enable/disable the auto-renewal settings.