Thanks everyone! I think I've figured out a good way to do this - it is a little risky, but not too much so IMO. What I do is create a long and unusual password, save it via PM on some of the sites that I'm planning on blocking and then block the sites on a timer. Thus, once the timer comes off, I can have access to the password and change settings/make a new schedule. This means I can't just circumvent it by putting in the password whilst the sites are blocked. There probably are other means to circumvent, but if they're too complicated, I simply don't bother.
Now it's just a matter of finding the right program for me. iNet protector is
almost perfect, if it were not for the fact that they offer to send your password via email if you forget it (I'm not going to forget it, it's saved in so many different places.) So I need something that
doesn't offer that option and also doesn't allow changes to the system clock/unistall. Oh, and something with a timer of course.