I will say this, and then nothing else. I have studied a great deal in my life, and I am still studying! I have studied most religions; a great deal of the occult. I have read Gardner,and Crowley,and Sanders, to name but a few. I have been a practising witch for over fifty years.
I have never, and I mean never, seen a demon; heard a demon; felt the presence of a demon. Why is this? Because something only exists if you believe it does. You know that certain things exist because the evidence is before your eyes. Demons do not exist for me, because I don't believe they exist. Much of all religion is such as this; it is belief. It you believe in goblins they exist; if you don't they don't!
I have been practising Ceremonial Magic for a long time now, evoking demons in the Goetia and being active in a community that has done for about 15 years.
The community is split between the demons being real, and then them being a part of your subconcious, to me they are real as I have read so much about possessions etc, but I have never seen a demon manifest other than shapes in the incense which was a hooded figure and then a sea turtle.
I have only spoken with 2 people who have seen a demon, 1 saw this in a srying mirror which again can be argued it was imagination, and the second saw one in his minds eye which is imagination.
Demons can't hex you, they won't hex you, they can mess up your life but only when you mess about with them or don't follow through with an offering you promised.
I agree with the above, demons dont genrally do that in my experience. Its likely that its a thought form either created by youself or another. The worrying about it feeds it so firstly do a good banishing(some good ones on here) , after youve banished have a good laugh, like the funnyest joke youve heard (forced if you have to) laughter is good weapon against them. Then do a protection ritual.
Invoking a benificial diety is also a good way, positive to the negative......