Is it possible to block out reality completely?
Yes, it is. Seeing objects, colors, hearing voices and sounds are electrical impulses translated by our sensorial organs and sent to the brain. There are cases of blind people which can "see" (feel?) a whole load of abstract images, and are actually bothered by it.

A person could have no sensing organs and still have electrical impulses by means of artificial brain stimulus, or due to a neurological damage; tinnitus comes to mind (no pun intended).


  When TMS [transcranial magnetic stimulation] is applied with a single pulse to motor or sensory cortex, it is possible to evoke a motor or sensory response. When the sensory cortex is visual cortex, the response is a phosphene.4 Phosphenes are the sensation of light in the absence of visual stimulation.