I don’t think I’ve ever heard a Pagan of any description speak of being “possessed” – at least not in a negative sense. The only “possession” Pagans usually experience is ritual possession, as in Drawing Down the Moon – and that’s generally a very positive experience. It’s also a temporary experience, presumably because our deities are busy and have better things to do than hang out inside us once they’ve delivered whatever message they wanted to deliver.
I have experienced ritual possession, once. It took a perfect atmosphere, a circle of trusted friends, and a skilled priestess to get me to relax my control enough for the Lord of the Animals to speak through me. Even while it was happening, it was all I could do to stay relaxed and not close myself off. And I didn’t want to close myself off – it was an awesome experience.
If it took this perfect setting to allow a possession that I wanted to happen, what chance would an entity that I didn’t want to let in or whose existence I prefer to ignore have to possess me? I’ll admit I’m more of a control freak than most people, and I like to think I have a stronger will. But I’m not that unusual. Remember: neither our gods and goddesses nor the demons of Christianity are omnipotent.
What I have heard plenty about are troublesome spirits and discordant energies. They tend to be isolated to a specific location, and although some do pursue an individual person (think back to the Greek Furies) they don’t possess them.
Thus, the likelihood of a Pagan leader being asked to perform something resembling a Christian exorcism is pretty low.
What we are asked to perform – and what every magically-oriented person should learn to do for him or herself – are cleansings, banishings, wardings, and spells of protection. There are many good references on these topics. Although she used a lot of Judeo-Christian imagery, Dion Fortune’s Psychic Self-Defense (1930) is a classic everyone should read. A more modern reference is Kerr Cuhulain’s Magickal Self Defense: A Quantum Approach to Warding (2008).
There are many others – these are my favorites. These skills are not hard to learn, but like all skills – magical and mundane – they require practice to learn and much practice to learn well.