Are funds acquired through fraud and identity theft, which their owners refused to reclaim after learning the truth, considered permissible (halal) and may one benefit from them?

What you have done by lying, deceiving, and wrongfully taking people's money is forbidden, and it is among the major sins. You must repent to Allah Almighty by abandoning the sin, regretting it, and resolving not to return to it, and by returning the money to its owners or seeking their absolution. If you obtain absolution from the owners of the money, and they willingly agree to leave it for you as a donation out of their own good will, then there is no harm for you in benefiting from it. However, if they paid you money in exchange for something forbidden, then this money is not lawful for you, and you must dispose of it by spending it on public welfare.