Don't follow people with more than 5000 followers.

Sure. Unfollow when you see one.

I definitely see posts from accounts I’m not following.

Are you from 2012 or something? Instagram has been showing promoted posts from accounts I'm not following for ages.

Those are just ads. The posts up thread making a distinction between ads and sponsored content I take to mean influencers shilling company products/services directly in posts outside of the Facebook ad network.