On Tristatevodou, one poster asked about Vodou countermeasures to Spiritual Oppression. Here is my response:
That's a great question. While Vodou has become (in)famous for "Vodou curses" and the like, in practice Vodou is far more interested in defensive than offensive magic. Houngans and Mambos spend a great deal more time protecting themselves and their congregation from negative magic than casting it on people. There's more demand for removing curses than for sending them. Keep in mind that Haitian Vodou has a strong animist streak: the universe is seen as a sentient and not always friendly place. Many times deaths are believed to be caused by poison or malevolent wanga -- and it is believed that the best way to avoid these is to have a strong team of spirits working on your behalf.
One of the great protections against malevolent wanga and oppression is the kanzo ceremony. When you kanzo it is believed that you have died and been reborn: because of that you gain a great deal of resistance to magical attacks. It becomes much more difficult to steal your soul because it's been protected (in ways which I can't discuss in greater detail, unfortunately). Spiritual oppression and obsession can be seen as a sign the sufferer needs to kanzo. This may not be seen as "demonic possession" but rather as a family spirit acting out to get your attention. Another ceremony which can help with your problems is the lavé tet, a ceremony which feeds your head and can remove spiritual parasites.
The gad or garde is also used for protection. There are gardes made by rubbing certain powders into shallow cuts, and a gad migan (stomach garde) which is believed to provide protection against poison. One famous gad migan involves the blood of a pig sacrificed to Ezili Danto. This is mixed in a basin with Florida Water and a few other ingredients and then each person in the house drinks three teaspoons of it. It is believed that after this you will vomit up any poison you consume: for a few days afterward you may well feel like vomiting up your lunch. ;)
And certain lwa and spirits are seen as especially powerful protectors. If you feel like you are suffering from spiritual oppression, you should definitely petition St. Michael the Archangel. If he can kick Lucifer's butt out of heaven, he can probably run off anything that is troubling you. And you need not be a Vodouisant, or even Catholic, to ask for St. Michael's aid: as an archangel, he will answer the call of anyone who needs his help.