What is the ruling on working as a computer instructor in a training center, given the possibility of trainees working in banks or other forbidden activities, and the inability to choose the trainees?

It is permissible to work in training unless it involves assisting in a forbidden act. If it is highly probable that the trainee will use what they learn for a forbidden act, then it is not permissible to teach them, based on Allah's saying: "And do not cooperate in sin and aggression." If you are compelled to teach someone whom you know will use their knowledge for something forbidden, then you must refrain from doing so. If that is not possible, then you must leave the job. For whoever leaves something for the sake of Allah, Allah will compensate them with something better, as Allah says: "And whoever fears Allah - He will make for him a way out And will provide for him from where he does not expect. And whoever relies upon Allah - then He is sufficient for him."