A psychic curse works similarly to a psychic blessing, but of course with a different intention.
The idea behind psychic curses is to entrap the consciousness of a target person at a specific level of negative psychic vibration. A psychic curse can target a place or item as well; in this case it works by saturating that place or item with negative 'sticky' type of psychic energy.
If the consciousness of the psychic doing the cursing is higher then the present state of the person being cursed, then the curse can be effective - but generally then there is no motivation to harm a person. If the consciousness of the curser is at the same level or lower than the accursed, then there can be a lot of motivation to harm, but the effectiveness of the curse is dubious at best.
In order to become immune to such psychic curses - strive for a clear state of consciousness at all times. Cursing someone or something because of a warped sense of justice does not befit a responsible psychic, and just causes unnecessary suffering on the inside and the outside for all parties involved.
In a clearer form a psychic curse can serve as a warning rather than a clumsy tool of hate. In this case, specific arrangements of psychic energies can serve as a gate to repulse uninvited guests, basically a psychic's way of saying: "This place is dangerous for the uninitiated - if you don't know what you are doing, go away."