The act of excision is to deny something's existence wholly and utterly. It is a way of convincing one's mind that something has never existed, or refusing its belief in the first place.
Countering excision is an interesting dilemma, because excision is an internal discipline, with no effect (whether real or imagined) on the excised object. An excised object may even attempt to interact with the one who excised them in the first place -- while there is nothing to prevent the excised from physically harassing the user, effects of their actions -- tripping, physical or mental assault, etc. will not be attributed to anything. Indeed, those who practice excision are able to deny one's existence so thoroughly that their being has developed a sort of "blind spot" for the excised. For instance, were an excised being to cause the user to trip or fall, the user would blame their own inherent clumsiness, or the wind, or any other number of things that defer them from the truth.
The denial of excision goes beyond a mere perceptual absence -- in conversation with others, references to the excised will be misinterpreted, or even flat-out ignored, understood as similar or sometimes even seemingly random language instead. Contributions of excised individuals will become anonymous, or even taken as natural miracles.
Excision is a form of punishment, and is not meant for, nor even particularly useful for, direct antagonism. An extension of "exile", excision can be much more traumatizing for the individual involved -- it is one thing for someone to be consciously ignoring you. It is quite another to forget they ever existed.
This was first used by Isura, though later Dajjal copied the technique (ironically, to remove Isura from her thoughts forever -- an action that would eventually lead to her death).
The only mortals in recorded history to use Excision was a group of Illmenite and Eladrin monks, from the Monastary of [blank].