Essentially, the two verbs mean to prevent someone doing something, not allow or prohibit. However, the difference between the two is when something was prohibited. If you are a member of a group but you were excluded, you are no longer a member. In opposed to preclude which means to excludes the chance in advance or not even taking something into consideration. To exclude also refers to expulsion (from the Church, for example). On the contrary, to preclude means to make something impossible, to prevent something in advance to happen. In other words, preclusion removes even the possibility of something to happen.
