First of all, I will teach you how to block someone and then how to unblock the same person, the requirement for you to block a person is that both users must have a Facebook account. Now if you have a Facebook account, follow the steps below;

How to Block Someone on Facebook
1. First, put on your data connection.
2. Next, locate your mobile browser and launch.
3. From your web browser navigate to facebook.com.
4. Now log in to your Facebook account.
5. After logging in, click on name downward arrow and select settings.
6. Click on blocking.
7. Hit the field where a name will be taken.
8. Input the person’s name and hit the block option.
9. Now you will be prompted to block the person when you are prompted click on the block option to block the person.
https://bitcoinometrics..com/2019/03/unblock-someone-on-facebook.html

How to Unblock Someone on Facebook
1. Connect your device to a secure internet service provider.
2. Next launch your mobile browser.
3. Navigate to facebook.com using your web browser.
4. Log in to your Facebook account, and click on the arrow pointing downwards from the top navigation.
5. From the new menu select “settings”.
6. On the left navigation of the settings page click on “blocking”.
7. Locate the “blocked users” section.
8. Click the blue button that says unblock besides the person name to unblock the person
9. You will be asked to confirm the person you want to unblock.

source: https://bitcoinometrics..com/2019/03/unblock-someone-on-facebook.html

Blocking on Facebook means a person’s posts won’t enter your feed, your content won’t be served to them, they won’t be able to send you messages, tag you, invite you to events and groups, start a conversation with you, or add you as a friend. This walkthrough will show you how to block and unblock someone on Facebook.

https://www.novabach.com/how-to-block-and-unblock-someone-on-facebook/