Did you abort games often?
Did you let time run out for no reason often?
Did you intentionally disconnect often?
If any of these are the case then that's the reason. You can't remove it because you're not supposed to be doing whatever you did that led to your account being restricted.
It does not allow me to play, it gives me the message "your account is restricted for violetions of our Sportsmaship policy. you may directly challenge friends or create rated, open challenges using standard time controls. This restriction will be removes automatically as you play more games with good sportsmanship". It does not say what I have done wrong so that I do not repeat it again. Does anybody know why this happened? And how do I "open a challenge using standard time controls"?