Apparently, it's really hard (no pun intended).
Here's what one occult site says (see source 1 below):
"... even with all the attention paid to them, there never seemed to be a foolproof way of warding off these demon lovers. Sometimes prayer worked, sometimes exorcism and benediction, but in many cases, even these proved futile.
Ludovico Sinistrari, a 17th century Franciscan friar, author if "Demoniality", wrote:
' Incubi do not obey the exorcists, have no dread of exorcisms, show no reverence for holy things, at the apporach of which they are not in the least overawed . . . sometimes they even laugh at exorcisms, strike at the exorcists themselves, and rend the sacred vestments'."
Here's another opinion (source 2) . You might want to read the whole story to put the reply in context.
"Is There a Remedy?
So what’s the remedy for an incubus or succubus attack? Should victims go to a medical doctor for relief from sleep paralysis? Should they seek counseling from a psychotherapist or psychiatrist if the experiences are the result of some childhood trauma? Or, as one reader posted in the bulletin board, should they seek an exorcism? The best advice might be to first see a medical doctor and go on from there. Psychiatric help would almost certainly be recommended for cases like the woman who wrote the e-mail at the top of this article. But should an exorcism – as we enter the 21st century – ever be performed? In some extreme cases, a psychiatrist might not even object. Since the firm belief in demons could be somewhere at the root of what is probably a very complex problem for the victim, the belief that deliverance could be obtained by casting out the demons or denying their approach in the name of a more powerful God, might be a solution."