Blocking: If your instagram is on public and there are certain people you don't want looking at your feed go to their profile, settings and block user. Sometimes someone may have blocked you and you want to block them back but you won't be able to find their instagram account if they have already blocked you. The way around this is to find a post that they have liked on someone else's feed, click their name and you will go to their account, then you can block them.
I block the following:
Bots that are used to buy followers
People who have followed me and then unfollowed a few days later. What they are doing is using a bot to like and get your attention. When you follow them they usually unfollow in a few days. You can usually see this sort of person by the number of followers they have compared to following and how many posts they have up. These time wasters are nothing more than annoying, so I block them to prevent them following me again and doing the same thing. I use an app called Followers it's a way to track your engagement and also see who unfollows you. There are plenty of them on the market