how do you block certain websites from your computer?

Hello
 
I hope this is what you were asking
For example:
Block using Windows hosts file
The hosts file, a plain text file Windows utilizes for mapping IP addresses and hostnames, can be used to redirect a domain name back to the local computer, essentially blocking the desired website. It’s a simple process and a great way to restrict users from seeing content you don’t want them to see, regardless of what browser they use or the time of day. Keep in mind you must have administrator privileges in order to change the file, and you can always undo the change if need be.
 
 
-Enter “C:WindowsSystem32driversetc” in the Windows search bar or navigate to the folder by clicking your way through Windows Explorer.
-Double-click hosts and select Notepad when prompted to choose a program to open the file with. Alternatively, you can launch Notepad and navigate to the host file by choosing File > Open, and locating the C:WindowsSystem32driversetc folder in your computer’s directory.
-In Notepad, you will see several lines of text used for mapping purposes. Underneath the last line of text (it should say something regarding a local host), enter “127.0.0.1” and press the spacebar. Next, type the specified IP address of the website you wish to block on the same line. For example, type “127.0.0.1 http://www.youtube.com" to block all traffic from the popular video-sharing site.
 

 
 
-Click the File option in the top-left-hand corner, choose Save – do not change the name or save location – and ignore any warnings regarding editing the hosts file. Close Notepad when finished.
More:
http://www.howtogeek.com/122404/how-to-block-websites-in-windows-8s-hosts-file/
 
As well, there are other possibilities lock - router, firewall,parental control..