It's not that the powers are blocked, it's probably closer to that they just have no effect on the Infinite Class. Think of it this way: the Soulswords are all part of the wielder's soul given physical form. They and their attacks give off a specific uncopiable energy signature that the Infinite Class cannot neutralize without trying. That is why they are the only things capable of harming the Infinite Class. That's the best explanation I can think of.
@Jester: Also, no, your spirit weapons are not the same as my Soulswords.