Is the father obligated to compensate individuals who fell victim to his son's fraud, despite the father having warned them and disavowed his son's actions?

You are not obligated to guarantee your son's theft and fraud, as that is tied to his own responsibility. However, it would have been better for you to advise him, hoping he might desist from such actions. If that didn't work, you could have threatened him with expulsion and abandonment. If that still didn't work, you could abandon him if there was a benefit in your abandonment of him, and warn people against dealing with him. You should remedy this situation by bringing him closer so he can hear advice from you. Do not resort to expulsion unless it is the only way to change the evil and you are confident it will be effective with him, because there is a fear that expelling him might increase his corruption. The basis for changing evil and its levels is the hadith of Abu Sa'id Al-Khudri, may Allah be pleased with him, who said: The Messenger of Allah, peace and blessings be upon him, said: "Whoever among you sees an evil, let him change it with his hand; if he cannot, then with his tongue; if he cannot, then with his heart, and that is the weakest of faith." Ibn Al-Qayyim, may Allah have mercy on him, elaborated on the levels and degrees of changing evil in his book "I'lam al-Muwaqqi'in."