Windows only: The hosts file is the best way to entirely block a website or other net entity. You can edit it by hand, but free editor HostsMan verifies, cleans up and manages your hosts file clean and simple.
HostsMan provides a human-friendly interface to that all-important text file. The editing app checks your entries as you add them and can scan your hosts for duplicates, possible hijack manoeuvres placed there by bad software, as well as back up, duplicate and temporarily disable your hosts file. It is, in other words, a smart tool to have if you'll be making a lot of hosts edits.
HostsMan is a free download for Windows systems only.