Is it permitted to “steal” someones internet device if I know they are doing aveiros with it? I am the only one who knows about the device and the person doesn’t know that I do.
Let’s be practical here. Stealing his device is not going to stop him. The same way he managed to get the first one, he’ll get another one. On the other hand, you do have to look for a way to help this person out with his problem. It would be a good idea to look for a person that is levelheaded, and will be able to speak to this person, and help him out. Not someone that will get all upset, and kick him out of yeshiva or punish him, but someone who will work with him, and help with his underlying issues etc.