Is it permissible to take something an owner uses for unlawful purposes without his knowledge and dispose of it, and if I have something belonging to a person whom I know will use it for unlawful purposes upon its return, should I return it to him, and would I be sinful if I did so?

Permissible things that are suitable for use in both lawful and unlawful matters may not be taken or destroyed without the permission of their owners. If they are with you as a trust or a loan, it is obligatory to return them to their owners whenever they request them. It is not permissible to refuse to return them on the pretext that they might use them for unlawful purposes, for the sin is upon them if they do so.