Unjustified curses are never as powerful as when they are justified. This is because in order for a curse to work, the object of the curse has to resonate in sympathy with the curse on some level, usually subconsciously. But when that person truly believes they do not deserve the curse, they will resist its vibration and the effect will usually be much weaker.
It works like this: the person performing the curse creates that vibration in her subjective universe as in any magical working (which I have at times referred to as “raising energy and putting it into a container”). The vibration is sent through the Magical Link into the objective universe, where it waits at the periphery of the target’s aura, looking for a way to affect his subjective universe—the sum of all of his inner selves. As with any type of magic done to influence another, the curse will vibrate in a certain way and attempt to cause a corresponding vibration in the target.
The third Hermetic Principle—that of “Vibration”—given in The Kybalion notes that:
All manifestation of thought, emotion, reason, will or desire, or any mental state or condition, are accompanied by vibrations, a portion of which are thrown off and which tend to affect the minds of other persons by “induction.” This is the principle which produces the phenomena of “telepathy”; mental influence, and other forms of the action and power of mind over mind, with which the general public is rapidly becoming acquainted, owing to the wide dissemination of occult knowledge by the various schools, cults and teachers along these lines at this time. Every thought, emotion or mental state has its corresponding rate and mode of vibration. And by an effort of the will of the person, or of other persons, these mental states may be reproduced, just as a musical tone may be reproduced by causing an instrument to vibrate at a certain rate. (http://www.sacred–texts.com/eso/kyb/kyb11.htm)
In this way, a curse will function by creating its vibration in the target, who will then radiate that vibration (usually without knowing it) into various aspects of his life. All retribution and judgment begins with the Self or, as The Kybalion puts it in “The Principle of Mentalism,” “All is Mind.”
If the target, truly in the deep core of himself, believes he doesn’t deserve to suffer, he will resist the curse vibration on some level. But, of course, to resist something is also to have a relationship with it. This is how an unjustified curse can make mischief in the life of an innocent. Essentially, by resisting, the innocent (consciously or unconsciously) accepts the reality of having to resist a curse and is then struggling against it and feeling some of its effects. But those effects are usually short-lived, if still upsetting.
When the target has a damaged aura—i.e. a personality defect, a mental illness, a negative attachment, or an inner weakness—a curse will try to use that as a way to influence the target’s vibration, working through the place in his aura where it is harder for him to resist. Magical workings (and curses are not exceptions to this) have a kind of intelligence to them—a guidance system that has caused some magicians to conclude that workings bring spirits to bear (whether the spirits are created, as with servitors and egregores or simply attached to the work).
If a target has an unusually strong, resistant aura, a curse can be held at bay for a long time, sometimes even for decades, depending on the longevity of the curse and that of the target. In the end, however, most curses will have to be dealt with somehow as only the weakest “blow over” on their own. Legitimate curses (i.e. those not formed inadvertently through the psychic capacity of someone feeling extreme anger or hatred for another) can be complex works of magic with many contingencies and resources involved. Like all magic, cursing is an art and should be taken seriously by those who feel they have been cursed.
The best way I know to deal with a curse is to have a competent spiritual worker cleanse and protect you. Using our Hermetic model, this would amount to a magician, sorcerer, conjure doctor, shaman, priest, or other practitioner performing work that raises your vibration above that of the curse, causing you to radiate a beneficial energy and heal yourself. Just as someone can be influenced to vibrate negatively, he can be influenced in the opposite.
For those who know what they’re doing spiritually (and / or who many not want to pay someone else), here are some texts on magical protection: Psychic Self-Defense by Dion Fortune, Protection and Reversal Magick by Jason Miller, Magickal Protection by Damon Brand, The Witch’s Shield by Christopher Penczack, and Practical Protection Magick by Ellen Dugan I am a bit skeptical of the last two texts on this list, but everyone has a different experience with magical books and you might find any of these helpful.