There are some people that I don't want to use this web site with. I don't want to see their questions, answers, comments and I don't want to let them comment or answer my questions. This doesn't have to mean that this users are bad users. Can I block them? If yes, how?
I don't know if there is an "ignore user X" feature, but if there isn't, it seems like something that could be done by a script running in the (ignoring) user's browser, that does a text replacement on unwanted usernames when displaying the page.
[Edit: a Greasemonkey Ignore Users Script has been on Meta.Stackoverflow since July 2009 ]
I don't want to let them comment or answer my questions
Chances of this ever being possible are zero. It would be a huge change to the site model.
It is very much intentional that you can't ignore specific users. While I understand that is might be useful in some cases, such a feature would be harmful to the site at large.
Ignoring a user doesn't solve any problems, it is just a fancy way of looking away or sticking your head into the sand. The behaviour that leads you to ignore that user won't go away, you just won't see it anymore.
Now, if the user is behaving according to the rules and the reason you want to ignore him is because you disagree with him, then this is a case that should not be supported by the software. Being challenged by people with different opinions is a good thing, and SE should not enable users to hide feedback.
If a user is disruptive or abusive, ignoring that user won't solve the problem. He'll still abuse other users and looking away is counterproductive. Flag the abusive behaviour for moderator attention and the user will be dealt with.