Unfortunately you can't turn off auto-complete for some addresses -- it's either all or none. However, as a workaround, you can move the old email address from the "Email" field in the contact record to the "Notes" field.
Using this workaround, you can still save the old email address for reference, and it will still come up in search results (in both Gmail and Contacts) but Gmail's auto-complete won't try to insert it when you compose an email.
To move an old email address from the "Email" field to the "Notes" field:
- Go to your Google Contacts
- Browse or search your list of contacts for the contact you want to change
- Click on the pencil icon ("Edit contact") to the right of the contact (it will appear when you hover over the contact)
- Cut the email address from the "Email" field
- Paste it into the "Notes" field
- Click Save