With Internet Explorer it's simple: you go to the settings menu, add websites you want to block, just randomly enter a sequence of letters and numbers so you cannot ever know the password, and it's done. (You could uninstall and re-install Explorer, but that's too big a pain to actually do that.)
With Firefox, its a different question. The main method is to add certain add-ons, like blocksite. However, add-ons like this have one FATAL flaw: you just deactivate the add-on and restart Firefox (which in all takes just a few seconds), et voilá... Too easy a by-pass to block you from not visiting certain websites.
I need to work on my bloody thesis, and seriously need to block like 10 websites. HOW?! Please help.
PS: can't have anything to do with the router, since there are several people using it, and adding URLs in the HOST also ain't good enough...