For this you need to know two things, your IP address and the login details to your router.
If it's your own WiFi, you simply put your IP address into a browser, which takes you to your router's settings. You enter your login details. Btw I think it's often "admin" by default for both login name and password. The settings should have an option that's like "Parental control" or something similar, which has a "Website filtering" setting part. You can add or remove websites there.
However if it's a public WiFi, you probably can't login to that if the sys admin has at least half a brain cell.
Is it just me who can't access this site using wifi?