Can a noise generator block a listening device?

There are quite a few ways that a noise could be stopped from reaching a listening device. In the case in your question, with a noise generator, it might work in two different ways - Destructive interference - The noise generator samples sound from a fixed distance, and creates it's own sound in 'anti-phase' of the sound it is sampling. With the correct calculations and properties, this anti-phase sound would cancel out the original sound. Overloading the device - Much in the same way being in a crowd of noisy people makes listening to someone next to you much more difficult, you could have a directed sound generation at the listening device, overwhelming it. This method is more brute force than the aforementioned. There are problems with both of these of course. Destructive interference is very hard to regulate, as depending on the distance of the source, the anti-phase signal will have to change. So the sound you were covering up would have to originate at a more or less stationary point. This can be solved by having multiple anti-phase points, but it's messy and gets very expensive. With regards to overloading the source, you'd have to know where the source was, unless you just wanted to blast the entire area!