Block from the person's profile. It works in both directions, immediately:
- Neither of you sees the other's posts, questions, requests, events or comments anywhere on the site.
- Neither can message, follow, mention or offer help to the other.
- They are not told. They will notice they cannot see you, which is not the same thing.
Unblock from the same place. If someone is doing something that breaks the rules, block them and also report them: blocking protects you, reporting protects everyone else.