Go to the computer you want to block and in elevated command prompt type in:
ipconfig /all
find out the MAC address of the machine
Logon to the router config page.
If you know the username and password, put it in. If you don't know or haven't set one up, search on google for default username and password. If you forgot the password, press the tiny reset button to reset and use default password.
In the LAN or wifi config page (depends on whether you want to block a LAN or wifi connection), go to the MAC address filter and block the MAC address.