Changing the point of the discussion to what seems to be the actual issue. You mentioned this:
"... the same user has first questioned, waited for two day - got some answers and then gave his own answer, after this he selected that as best."
However, I find absolutely nothing wrong with that. In fact, I've actually done that myself (http://www.question2answer.org/qa/30898
) and I'm not embarrassed at all. Furthermore, users can benefit from my answer too.
As a side note, assume a feature to disallow giving points to users selecting their own answers was implemented. Now, consider that users can still take the same steps regardless of the fact that they won't get any points. For example, they could do this to have the selected answers count in their profiles increased. So you're not really removing the cause of the issue, if there is any issue at all.
So if you think there is something wrong with the attitude of some users probably it would be better to remind them of the TOS, which I guess have something like this covered. If not, maybe adding a "Users must act in good faith" clause or something like that.