Start by going on the profile of the person you want to block. Then click on the three dots top right. Then click on "Block this user" section.
Once you blocked someone, the user won't be able to find you on the app, will no longer have access to your profile or any of your contents nor the comments or your likes.
By going to your settings on your profile (top right), you will find the list of users you blocked and choose to unblock them if you want.