How does a person purify himself if he has consumed unlawful wealth, or wealth concerning which there is doubt, or if his father dealt in usury, and he only became aware of this after a period of time? And does the Hadith, "Every body that is nourished by unlawful means, the Fire is more befitting for it," apply to him?

There is no sin upon someone who eats or wears something unlawfully acquired while being ignorant of its true nature, nor is there any liability upon him. The default assumption regarding what is in a Muslim's possession is permissibility. There is a difference between dealing with a possessor of unlawful wealth regarding the unlawful item itself and regarding the rest of his wealth. The former involves difficulty (haram/prohibition), while the latter does not prohibit dealing with him in it. If the unlawful wealth pertains to someone's right, then the right must be returned by the wrongdoer, not by the ignorant person. Some scholars are of the view that wealth which is unlawful due to its method of acquisition, not due to its inherent nature, is only forbidden for the one who acquired it.