How to delete prevent remote access to my iPhone

People do not have remote access to your iPhone.

However, you should take care to safeguard your Apple Account, and to safeguard the local credentials (iPhone & iPad passcodes, Mac passwords) of any Apple device associated with that Apple Account.

If you were synchronizing photos through iCloud, somebody who knew your Apple Account ID and password could access your data using another device or even a Web browser. They could "put photos on your phone" or "delete photos from your phone" by deleting the ones on a synchronized device, or in iCloud – following which your phone would copy the changes, even though the attacker had no remote access to the phone itself.