Removing a hacker or remote user from my iPhone

Extremely unlikely (impossible) to be hacked like that unless you let someone access your open phone for some time.

However you can move all your important documents and photos to iCloud or computer then DFU the phone and start again.

Back up the phone just in case but don't ever use the backup unless you find your fears were unwarranted..

(1)Backup

 How to back up your iPhone, iPad, and iPod touch - Apple Support

(2) Change your Apple ID password

Change your Apple ID password - Apple Support

(3)       Recovery mode or DFU to refresh the iOS

 If you can't update or restore your iPhone or iPod touch – Apple Support (UK)

Set up the phone manually with your Apple ID and start afresh. Import your documents and photos and download all the apps you need. Use a six character phone passcode.