lets me to wrong websites
When I quit smoking cigarettes, one of the techniques I used was to make it more difficult for me to have a cigarette.
For example, if I owned a packet of cigarettes, and the craving occurred, then it was easy for me to have one and I did.
One technique for making that more difficult was to not have a packet of cigarettes. So when the craving occurred, I didn't have one available, and to get one I would have had to walk to the shop etc. That walk (which would have lasted many minutes) gave me time to rethink my craving, or enough time for the craving to go away. It was more of an effort, and gross motor rather than fine motor activity. Although I could not control whether the craving arose, however I could control whether I acted on it or gave into it: I could decide or remember that there was a reason (that I had had a reason) why I didn't own any cigarettes, and I was able to decide not to put my coat on and go outside and walk to the shop and buy more and smoke more and continue that cycle.
There were some times, while I first quit, when I literally couldn't think about anything else except for wanting to have a cigarette, and (having decided not to go to the shop and buy some) I just had to stand there and wait for that craving to pass and for a next thought about something/anything other than that to arise.
So if you want to stop visiting some web sites, perhaps you can do the same, i.e. ensure that those web sites aren't easily available to you: for example install or enable "parental control" software on your computer, or edit the etc\hosts file (see e.g. here and here) to make the site unreachable from your computer. It's true that if you change your mind, if you decide that want to access the web site, then you can undo whatever you did to block the site ... but the fact that you need to undo/unblock it makes it a little bit harder, and acts to remind you that you had blocked it for a good reason.
I suspect that's true for most addictions: for example if you're a drug addict, it's easier to quit (and harder to continue) if you decide to move to a different environment where drugs and drug-using fellows don't exist.
In summary you might find it helpful to create a safe/insulated environment (or web browser setup) for yourself where temptation is less available or completely out of reach.
Two other suggestions: one is you might find it useful to log how you use your computer (for example, look at your browser History some times) ... that's on the theory that "you can't manage what you don't measure" so you measure/monitor your use of the web in order to manage it better; the second is that it's not a bad idea to do things with good friends ... they're likely to inspire or lead you towards virtue (e.g. you might think, "he is good and behaves well, I too can do what he does").
These are mechanical/physical suggestions, not just an idea to keep in mind, because you said you you want a practical solution for even when "thoughts come again".
I think you were hoping for an answer which didn't "sound very theoretical"; but you might wonder how this answer related to Buddhism.
Firstly, when I wrote the answer I was reminded of this topic: “Guard the sense doors”- What does this mean, and what is it's application?. That topic include, for example:
Guarding the sense doors is a way of protecting ourselves from being overcome by passion for the sensed object (which leads to dukkha). In his video Yuttadhammo mentions three ways to do this.
- Physically avoiding the object (in relation to your example, when on alms round monks may keep their eyes facing down to avoid lust from seeing a beautiful woman).
So a mechanism which physically "hides your eyes", puts a barrier in place, might be an effective solution.
Secondly, there's a Buddhist doctrine of the "Twelve Nidanas", which teaches that what you think and feel and become attached to is conditioned by your senses, and specifically by contact between the sense-organ (eye) and sense-object (web site on a computer screen). Again that suggested that if you can't control your thoughts and attachments (which are conditioned by or which arise as a result of contact), perhaps you can control whether there's contact (again, by putting more of a barrier or filter between the sense-organ and sense-object).
Thirdly, Buddhist often choose or recommend seclusion or "a retreat": to avoid distraction etc. You might want to adopt a similar attitude -- especially on Uposatha days.
Lastly I'm reminded of this sutta, which describes how to overcome torpor -- not the details of it, but the strategy, the structure: "Do this: maybe this will work. But if doing this doesn't work, then do that. And if doing that does work, then ... and if that doesn't work, then ..." and so on, 7 or 8 different methods. So, whatever you previously tried (e.g. contemplating the body as a Bag of Bones), if that was too theoretical, then try something else!
Back to my personal experience, my experience is that it might be easier to control your (my) behaviour than thoughts: thoughts happen (uncontrolled) but you may be able to control (or decide to not act on) your reaction subsequent to those thoughts. Similarly it may be easier to control large movements of body over a longer time period (e.g. do you or don't you go for a walk) than small (hand or face) movements which you might do without much thought.
You're right though, it's kind of alarming to discover that you "can't control yourself". Buddhism sort of warns you about that, saying for example, "don't call thoughts my 'self' because if they were my 'self' then I'd be able to control them". Still it is possible to change, to learn to change, a habit (habitual behaviour).
