Will I be sinful if I work for a company that provides internet service, and one of the subscribers uses it to access pornographic websites?

The internet contains both good and evil. Working to connect it to homes or designing websites for it is permissible, as long as it is not highly probable that it will be used for disobedience to Allah. However, if it is highly probable that it will be used for disobedience, then it is not permissible to assist with it, based on Allah's saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." If the situation is unknown, then prevention is more appropriate due to its frequent use in ways displeasing to Allah.