According to Wikipedia, psychological horror is

Psychological horror is a subgenre of horror and psychological fiction that relies on mental, emotional and psychological states to frighten, disturb, or unsettle readers, [...] and it often uses mystery elements and characters with unstable, unreliable, or disturbed psychological states to enhance the suspense, drama, action, and paranoia of the setting and plot and to provide an overall unpleasant, unsettling, or distressing atmosphere.

And further on:

Thus, elements of psychological horror focus on mental conflicts. These become important as the characters face perverse situations, sometimes involving the supernatural, immorality, murder, and conspiracies.

In my understanding of the genre, the focus of this kind of horror is making the audience face something scary that's closely grounded, on related, on the human psychological condition. This may be "grounded in reality" - e.g., having to deal with depression, anxiety, situations of great fear or stress and so on - or portraying the same conditions in a more simbolic way.

Take for instance the film The Babadook. In the movie, there's a shadow-like monster tha haunts the house of a recent widow and her son. The monster is a literal monster,

until we discover that it's actually a representation of the mother grief for the death of her husband, and the fact that the child, who never knew his father, will have to grow up "dealing" with this feeling of grief.

Coming now to my question: how can one ensure, when writing a psychological horror, that any supernatural/paranormal or just very unlikely element is not taken face-value?

If I write a shadowy figure, I don't want it to be the next Slenderman. This is of course related with symbolism, since those creatures should be "symbols" for the psychological dread of the genre.

Of course, a lot of novels can be read on various levels of meaning. Lovercraftian stories may be read as a lone protagonist losing his mind or a parable about existential vacuity. But some are more apt to be interpreted than others.