Synchronizing my phone with my gmail account it has imported, not only the contacts, but even all the people who I just wrote one mail in my life (even without phone number). I have hidden this contacts checking "show only contact with number" but if I perform a search this contacts appear. How can I really delete it, and set my phone to only consider GMail contacts (https://mail.google.com/mail/u/0/#contacts).
From my experience, GMail won't put people you have only emailed into your "My Contacts" built-in group (See Gmail and auto-adding contacts).
Because of this, you should be able to tell android to only pull from your "My Contacts" list, which would presumably only contain your important contacts.
The steps to do this will vary depending on your ROM. I'm using a nightly build of CM10, and these are the steps:
Hopefully that helps you out, or at least gives you an idea of how to accomplish this.