First things first: If you really do work to overcome the following system, you really (no joke) should seek psychological assistance (note from the future: I did seek such assistance and it helped me greatly. If the tool recommended won't help, consider writing me an email).
I, as you, am (kinda) addicted to specific sites which I visit often and spend far too much time there. For this recommendation, I'll just name it tvtropes and will use that instead of example.com.
I use Firefox/Iceaweasel as my only browser, so this is a recommendation for a plugin for this browser. If installing a different browser is something that you count under "just disable it" then this will be not much use. But, bear with me, because it has some nice features that help exactly people like you and me.
But for this I need to point something out: The reason why I (and likely why you) find the sites so addictive is because they are a click away and then we can just enjoy the endorphines of consuming their content. If we are forced away from them, withdrawal comes and haunts us. So, if we can find a way to remove them slowly its far easier.
I recommend: Leechblock
It allows you to blacklist all the sites that you love too much and then lets you define rules for them. For example: You may visit all of the sites, but every 2 minutes when you open a subsite you have to wait 30 seconds for it to open. If you haven't tried this yet, do so. Its very helpful, because if your really like that site /now/, you can have it. Just wait 30 seconds and there it is. You browse there and do what the site is there for but then, you again have to wait. This reduces the reward of the site to the point where you can start focusing on something else instead.
Now, you say, "I can enable it, I can disable it". Yes you can, but, its hard. You can even tell Leechblock how hard it should be. But it always will be more time than waiting 30 seconds. You could either fiddle with the files of your firefox or start installing a different browser or - whatever, but it will take more than 30 seconds. And thats why you will wait for the 30 seconds timeout (because it resets if you do something else on the PC. You have to wait.)
So, this is my setup:
- I have one group that contains the "everydayleech" like tvtropes. I allow myself 5 minutes of these sites every 4 hours from 08.00 - 17.00 with a timeout of 30 seconds. I may not change the rules while they are active.
- I have one group of sites that I occasionally need for my work, but spend far too much time one (stack overflow is one of them). I allow myself 20 minutes of these sites per workday but they only count when they are active. I may change the rules on them because sometimes I just need to.
- I have one group of sites that I may not change and they contain the links to sites where workarounds of leechblock are listed.
This setup has caused my timeloss from about 7 hours / day to about 20 minutes per day.
I hope it helps you too.