Can I disable saving passwords in Safari 8?
Your Mac or settings are broken for Safari to be locked like that. 

This is likely an issue with your user account settings since all the Macs I have allow you to uncheck those marks. First, try going to the Passwords tab and turning off all check marks there before returning to the AutoFill tab.

Before wiping your user settings and preferences, can you verify that making a new user on the mac and logging in to that account, you are seeing the same issue?

If it's a system issue the steps to fix it are radically different than if it's a user level issue.

See Isolating issues in Mac OS X and Mac OS X: How to troubleshoot a software issue