Just quickly dropping by, I might write something more in-depth later but I gotta get back to work :)
The most important thing, the ever-so-cliche, YOU MUST DECIDE. Your commitment must be strong enough. Without it, nothing else will work. There are people, like @Cam Adair of course, that can explain this much better. BUT... I've tried quitting some games before so I've learned some easy and cheap tricks that might help out a little bit. Mind that without your decision to quit, these external helpers will not make you succeed.
Throw away your gaming mouse. Literally. Throw it into a river. Make a statement. It's cheap enough so that you won't mind the money. Without your gaming mouse, you can't game. I also trust that you are lazy enough to not buy a new one, that actually takes some steps to take so in that time you can recuperate, change your mind and not buy it. I haven't heard anybody mention this, I just discovered that I couldn't game when my mouse broke.
Change the e-mail adress of your gaming accounts(steam, battlenet, LoL etc.) to the e-mail of a friend you trust that will not give it back even when you have cravings and might ask for it. Or even your parents. Then change the password to some random characters. Then uninstall everything. This way, you can't access it, neither can you retrieve the password.
Destroy your in-game collection. This works for almost any free-to-play game, I will use hearthstone as an example - this is something I ACTUALLY did years ago. Never played hearthstone since. Log into hearthstone, disenchant every single card. Then, in order to prevent you from logging in later and utilizing that dust, you're going to make a golden meme deck. Go through the most useless cards in the game and craft golden versions of it. My entire hearthstone collection is a single 30-card deck with golden millhouse, golden whisps etc. This effectively destroys your dust and you will never want to play again.
Ask the support to delete your account. This only works in some services, afaik steam and battlenet doesn't let you do that. But league does, for example. They can delete your acc or put a perma-ban on it if you ask them.
Sell your PC and get a MAC. Boom. You can't game now, but still own a computer for real-life things.
Install Ubuntu on your PC, allocate the maximum amount of memory. So that your windows will not have enough space to install any games on. This solution is similar as getting a mac except that you keep your PC and use linux instead of windows.
I thiiiink that's it. I will write a more in-depth blog post about these cheap tricks very soon.