Mediumship is beautiful and is an extremely useful faculty but, there are dangers involved. Below they are classified as follows from The Mediums Book by Allan Kardec.
1 - Simple Obsession - exists when a spirit imposes himself on a medium, against the medium's will, into and with all the communications received by the medium, or prevents the medium from communicating with other spirits, by substituting himself.

In this case, the medium is aware that he is dealing with a deceptive spirit.

2 - Fascination- is an illusion which is produced by the direct action of a spirit on the medium's thought, which paralyses the medium's judgment with regard to the communications the medium receives.

The deceiving spirit artfully inspires in the medium a blind confidence which prevents the medium from seeing the absurdity of his thoughts and any spirit communications received, even when this is clear to everyone else.

The fascinated medium cannot believe he is being deceived.

3- Subjugation- is the third degree of obsession in which a spirit paralyses the will of the medium, reducing the medium to a state of absolute bondage. Subjugation can be moral and corporeal.

Moral, in that, the medium is often drawn on to do things which are foolish or horrible, which the medium is deluded into thinking are wise and proper. It is a species of fascination which acts on the will and the mind.

Corporeal, in that, the spirit acts on the material organs of the victim, provoking involuntary movements or acts.

The spirit is in complete control of the medium.

We may recognize the existence of obsession by the following signs:
1 - the persistence in a spirit communicating, by any means, and his persistence in preventing other spirits from manifesting themselves.
2 - the illusion that prevents the medium from seeing falsity and absurdity in the communications he receives.
3 - a belief in the absolute identity and infallibility of spirits, who under respected and venerated names, say foolish and false things.
4 - The medium's confidence on the praises heaped on him by the spirits who can communicate through him.
5 - A disposition to shun the presence of those who might give him useful advice.
6 - Offense taken at criticism of communications received by him.
7 - An incessant desire to write, etc., without regard to time and place.
8 - Physical constraint over-ruling the medium's will and forcing him to speak and act, in spite of himself.
9 - Persistent noises and other disturbances around the medium, of which he is at once the cause, and the object.

Points to remember

Evil spirit influences affect people whether they are mediums or not. Knowledge of Spiritism makes us aware these influences exist and it helps us to combat these influences. There are no "magic" words that can be said to remove evil spirits from our side. Our moral imperfections bring us under the power of obsessing spirits, and the surest method of getting rid of them is to attract good spirits to us by the practice of virtue. Good spirits are more powerful than bad ones and their will suffices to keep the bad ones away. But… they only assist those who second the action of their will by their own efforts. If no such effort is made, they may withdraw leaving us free to the evil ones.

There are ignorant and evil people, when they return to the spirit world, their moral inclinations do not change, so they are the same as spirits. It is only through successive existences, through the law of cause and effect, and that of progress, that they will eventually see the error of their ways and advance in morality and goodness.  

Reference
From The Mediums Book Chapt. XXIII