Get rid of Hacker

Unfortunately this is not totally correct. iPhones, just like any other phone, can be hacked many different ways. There are several Zero-Day hacks that can be sent to Messages alone that will hack the phone without the user even knowing about it.

I wouldn't be so cavalier with saying that iPhones absolutely cannot be hacked remotely.

Both you and the original poster should listen to the podcast SecurityNow by Steve Gibson. Almost every week he is reporting on different ways both iPhones and Androids can be hacked remotely without the user even knowing.

In the end you didn't answer the question put forth by the original poster....will a reset of the phone (deleting all user data) remove any hacks introduced.

By hack, if you mean remotely controlling your device, then...

Keep the iPhone updated to the latest iOS always and never Jailbreak. That's it.

iOS / iPadOS devices cannot be hacked or infected with Virus / Malware / Spyware if it is updated to the latest iOS/iPadOS unless you have intentionally downloaded spurious software or unauthorized apps directly from the internet and installed on your device or/and have Jail Broken. 

It (Hacking) also depends on how careful are you in sharing sensitive and valuable information pertaining to your iPhone such as Passcode, Password, etc with your friends and family members.

Be careful when sharing the device's sensitive and valuable information with friends and family members.

Thumb Rules:

1. Enable  Guided Access before handing over an iPhone to kids
2. Don't share Apple IDs
3. Don't Jail Break
4. Don't share sensitive information pertaining to your device and payment methods
5. Beware of Phishing

BrianBlueeyes Said:

"Get rid of Hacker: Someone got ahold of my iPhone and hacked it they know where I am and are reading messages and broke into my house and car if I factory reset and update to latest version of iOS will that remove them from my phone?"

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Modifying your Credentials:

Having so much suspicion, consider modifying your credentials, ASAP, starting with a new password.

• Modify: your login credentials (using both a New eMail and a New Password) for Apple and each and all sites
• Associate: a different method of payment to your account
• Enable: Two-Factor Authentication for Apple ID PayPal and for all other accounts that have such an applicable option