Is it permissible to disable a computer after repairing and using it, for fear of it being used for unlawful purposes by others?

If the device is yours, you can put a lock on it to prevent others from operating it. But if it is not yours, you are not permitted to disable it, because that is an infringement on another's property. There is no sin upon you if people use it, and it is better for you to advise them and warn them against disobeying Allah and using devices in ways that displease Him.