There is no valid evidence that Dorje Shugden is a spirit, but one of the three reasons for the Dalai Lama’s ban on Dorje Shugden practice as stated on his website is:
Such practices degenerate the profound and vast teachings of Buddhism, wherein our ultimate refuge is the Buddha, the Dharma and the Sangha. While the profound teachings of the Buddha are based on the two truths and the Four Noble Truths, the appeasing and propitiating of Dholgyal (Shugden), to the extent it is done by those who do this practice, degenerate the Buddhist practice into a form of spirit worship.
If the Dalai Lama is so concerned by the possibility of Buddhadharma degenerating into a form of spirit worship why does he and his government in exile rely upon the State Oracle Nechung?
Replies to the Three Reasons for the Dalai Lama’s Ban on Dorje Shugden practice from westernshugdensociety.org