G-d gave us human beings the gift of imagination, something that no other life form has been endowed with. Without imagination we wouldn't be able to function as human beings, to invent, to progress and to search for the truths of the universe and for G-d. Yet, at the same time, imagination is responsible for most of the Yetzer Hara's power over us. The reason why we "lust" for things that we know we can never have and that are bad for us, is because we imagine that we could have them, we imagine that they are good for us.
R' Nachman says that the name of the Yetzer Hara in our day and age is "Dimyon" - Imagination. We need to learn to use this wonderful gift of G-d only for the right things. When it comes to lusting and fantasies, we need to turn off the imagination and internalize that all these imaginary desires out there have no connection to us. For in reality, having all these imaginary desires would be bad for us, bad for the people involved and bad for the world.
So next time you catch yourself fantasizing or lusting, start to train yourself how to turn off the gift of imagination sometimes, and live in REALITY instead.