To remove a contact from auto-complete, please tap More (o o o) | Edit Profile, Groups & Contacts | Contacts | Search for the contact(s).
From here you have two options. Either you can remove the contact from displaying in your autocomplete list or remove the contact entirely.
To remove the contact from appearing in the autocomplete bar, without deleting this contact – please disable Show this person in autocomplete.
To remove this contact completely, please tap the 3 dot menu | Delete | Yes.