I got associated to many Stack Exchange communities and I want to clean up sites I don't use. How can I stop being associated with, for example, Mathematics Stack Exchange but not the others? I have signed up with my Google account.
You can delete that account using the instructions mentioned in How do I delete my account? in the Help Center. This will delete it only on one site and won't affect your accounts on other sites.
Alternatively, you can hide one of your accounts without deleting it—see Hiding sites from the main StackExchange.com profile page?.