Blocking Methods for Facebook :
Tap menu in the top right of Facebook. (Three Bars)
Tap Settings & Privacy, then tap Settings.
Scroll down to Privacy and tap Blocking.
Tap Add to blocked list and enter the person's name.
On the search results page, find the person you want to block and tap Block.
Tap Block again to confirm your decision.
If Not Done By this method try the below one
If you can't find someone using this method, try going to the person's profile and selecting Block from the menu.
Hope it resolves your issue