What is the ruling on responding to a person's request if you believe or suspect that he will use the assistance for something forbidden, or for things that are permissible and others that are forbidden?

A distinction must be made between certain knowledge and strong conjecture (which is considered valid and upon which rulings are based) on the one hand, and doubt and illusion on the other. If one has certain knowledge or strong conjecture that a person will commit something unlawful, it is not permissible to assist them. However, in the case of doubt or illusion, the default is permissibility. As for repairing a computer for someone known to use it for both permissible and impermissible matters, it requires advising its owner and reminding them to fear Allah Almighty.