Photos will identify and group similar faces found in your iPhone's photos, whether they are a friend or a complete stranger in the background. If your People album has some unwanted entries, you can hide them simply by choosing "Select" in the top right corner of the People album, then tapping on the person you want to hide and selecting "Hide" at the bottom. The person will now show up in the hidden people section at the bottom, and you can choose to not show this section as well with the "Show less" option at the top of the section. To unhide somebody, go through the same steps listed above and choose "Unhide."
How do I hide a person in Photos' People album?