To not have an address show in the autocomplete, while typing an email address in the Gmail web application, the address needs to be deleted from the user's contacts, this can only be done by the user, if you want to do it, from the account that is showing that address follow these steps:

1. Go to contacts.google.com
2. Use the search bar to look for the address you want to stop seeing in the autocomplete.
3. Click on the result and use the three dots to delete it.