What I can gather from that site is that we are all born with purity and the knowledge of the oneness of god and a desire to submit to him. Essentially everyone is a muslim from conception and they are swayed from the true path later on.
Osman said:
Impurities cause the desire to do bad things.
Impurities will be cleaned away.
Therefore, the desire to do bad things and feel bad things will have been removed.
"The central hadith makes plain that it is the social circumstances after the birth of the child that causes the individual to diverge from fitrah. Hence if someone follows an aberrant path it is not because of any innate wrong within his nature, but because of the emergence of the lower self or nafs after birth, and negative effects in the social circumstances."
Not so much impurities within you, but ignoring the purity you were born with by choice.
"Although all children are born in a state of fitrah, the influence of the environment is decisive; parents may influence the religion of the child by making him a Christian, Jew or Magian. If there are no adverse influences, then the child will continuously manifest his fitrah as his true nature."
From this it seems that your ability to choose what you do or what you accept to believe determines whether you become a muslim or not and consequently your admittance to paradise.
Therefore in order to restrict a persons desire to do bad in paradise you would have to restrict their will, as their innate desire (fitrah) is to do good (in an islamic sense).