I've done it. I usually block toxic people in real life, who use FB for their nosy business and habitual-throwing- rocks-of-criticism at you in RL. and yeah, it wasn't easy.

I remember the first person I ever blocked. It was this guy, who was a couple of years younger than I was and he started to talk to me online once I had left for college. He started then stalking what I was doing when I was away and he kept telling me how he thought all the time about me. It made me feel very uncomfortable especially when he started demanding for me to be online and talk to him all the time. I simply blocked him. He created a fake account to talk to me again, but I understood right away and did not accept the request to chat. He kept sending emails about how unfair I was but I refused to even reply.

I usually do not block people, but if they make me feel the slightlies uncomfortable, then I do. This is probably the reason why to begin with I am very selective about friend requests.