OK, so I guess that's nothing you can do by yourself with your current knowlodge.
This project isn't only HTML, JavaScript and CSS. It would most certainly have a database and a back-end language.
My point is: what you want is not only to "create a block button", you need to create a user block functionality. And for that you need to adapt the database, program all logic in the back-end language and then, only then, create the block button in the interface(and probably a blocked users list or something like that).
If you want to do it yourself you will need to have access to the database, have all source-code with you, and last, but most important, a very good knowlodge in web development.