The short answer is: Yes.
The long answer is: An atheist is anyone who is not theist, and being theist is not a criteria you have to fulfill in order to be spiritual. Hence, some Buddhists may be considered atheists. Deists are their own category, but are technically “not theist,” either.
I don’t see why this is so controversial. In Thelema, you are your own God. There is no “higher power” that rules over you. You are not expected to anthropomorphize any concept of divinity or spirit. There’s a reason why we have sayings like,”Man created God,” or “God tends to agree with the believer.” The very projection of human frailties resulting into a theistic God is an example of the repressed shadow side and makes such a “god” into an authoritarian tyrant and the leaders of such believers into megalomaniacs and cultists. I’m not saying that there aren’t theistic Thelemites or magickians but the law of Do what thou wilt does not assume theism. Neither does ceremonial magick. Magick is a practice, not an ideology or a religion.
As for the existence of spirits: We know that everything is energy and that a physical body is not necessary for consciousness on other planes.
To paraphrase Motta (Astral Attack and Defense): So-called “demonic possession” only happens with the followers of the organized patriarchal religions. It is better known as mental illness; that is, the “demons” are repressed aspects of the personality and self-expression (including sexual). The supernatural simply entails the phenomena that has not yet been explained fully by science (i.e. today’s magick is tomorrow’s science). Gullible and superstitious people like to use the misnomer “supernatural” or claim they are being psychically attacked without even investigating their physical and psychological health. Yet there is nothing that does not follow natural law.
Of course, there are some magickians who proclaim to be Deist or even atheist, and believe in an ancient race of gods/angels that watch over humanity and alter the course of history. I’ll leave that topic for someone else.
In conclusion, one can start out as an atheist – or anything else, really. But what happens after that I can’t say whether or not they’ll remain the same or change their views towards divinity. If people treat magick as a process and not a thing, then it will be about their personal development.