All perfect praise be to Allah, The Lord of The Worlds, and may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
It is prohibited to use someone`s property save with their permission because the Prophet (PBUH) said in this regard: "It is prohibited to take someone`s property save with their blessing." {Musnad Al-Imam Ahmad}.
Moreover, Al-Shafie (May Allah have mercy on him) stated: "It is prohibited to benefit from someone`s property, such as picking fruits from an orchard or milking a cow, save after getting their permission since there is no Sharia text mentioned in this regard." {Al-Majmou`,vol. 9 pp.55}. Accordingly, it is imperative that you(Asker) seek the pardon of the person whose internet you have been using, and give whatever compensation they ask in return. This is in addition to making sincere repentance through showing remorse, stopping what you have been doing, and being determined not to do it again. You can also make good deeds, such as remembering Allah and sending prayers on the Prophet (PBUH).
However, if you couldn`t reach that person, then you have to estimate a certain compensation by asking some internet experts, and spend it for the benefit of the Muslims. And Allah knows best.