Article in English: Unsyncing an account will remove all of the contacts that were synced from it. This can be useful for removing a lot of contacts at once. You'll find this in the Personal section. Any contacts from this account will be removed from your device. This will turn off contact syncing so that the contact list isn't automatically updated with your contacts from that account. If you don't see a Contacts option, toggle syncing off for that account completely. You'll see this in the upper-right corner. A small menu will appear. This will sync with the account, and since contacts were disabled all contacts for that account will be removed from your device.

Summary in Vietnamese:
Nhấn vào biểu tượng Settings (Cài đặt). Nhấn vào Accounts (Tài khoản) nằm trong phần Personal (Cá nhân). Nhấn vào tài khoản mà bạn muốn tắt đồng bộ hóa. Chuyển tùy chọn Contacts sang vị trí OFF. Nhấn vào nút hình ⋮ ở góc trên bên phải. Nhấn vào Sync now (Đồng bộ hóa ngay bây giờ).