Hi zhenqing,
From what I understand, getting rid of spirit attachments is one of the natural outcomes of Working on ourselves, and like gaining knowledge and understanding, can also be done through normal means. Take the guy Laura mentioned that had an attachment that made drink whiskey and follow sports for example. If instead of having the entity sent away he practiced self-observation, saw his habits as not something he did not want in his life any longer and then struggled against long and hard enough through discipline, the attachment would simply leave because it no longer would be able to "feed" off of him. In this case, however, not only would he have lost something "bad" but from the struggle and experience would have gained something valuable that he could in use in his everyday life or towards a greater Aim. He would also be stronger for it and less likely to have another attachment come in and just replace it, which I think is what Castenada refers to as the "glowing coat of awareness that makes us unpalatable to the Predator", and could also be one interpretation of what Gurdjieff meant he said that when struggling against one "influence", we must be very careful not to allow ourselves to come under another in it's place.
For me the biggest lesson from this video so far, is that in a similar way to what Laura said about not having to "believe" in discarnate entities for the differential diagnosis method to work, in our struggles against habits and influences that conflict with our Aims, it doesn't matter whether our influences come from ourselves (programs, etc.), a dead dude or an elemental because each influence still takes the same amount of work to be rid of permanently (including replacements).
In more serious cases, or very strong attachments/influences, we may certainly need some breathing room from it before we can deal with it more fully. And for that, the diet and EE, among a few other things, are really good starting points :) I'm sure the C's themselves said that EE is the gentlest and easiest way (or something to that effect) to help cleanse ourselves from these kinds of things.