Is it possible to completely roll your eyes?

No: No: if what you mean by this is to roll your eyes back in your head and "look" at your brain. The eyes are firmly secured in the eye sockets by ligaments and muscles which suspend them and move them in all directions in the frontal plane only. Additionally, the conjunctiva and underlying tissues surround the eyeballs and connect to the eyelids and front walls of the sockets tethering the eyeballs. Finally the optic nerve travels back from the back surface of the eye to the brain and only has a little stretch but does not allow total backwards rotation of the eyes.

No: The eye is not a simple ball; it actually has a long tail (the optic nerve) that tightly attaches it to the brain, as well as 6 muscles that limit movement. These together prevent the eye from rolling more than a cuple of inches in any direction.