My wizard summoned it, and sometimes it messes up fights. But there doesn't seem to be a way to dismiss it. I tried to let my cleric used Dismissal spell on it, and there was red text "Invalid action" popped up.
Nope. And I have also found that, even elemental gems don't have such a choice in the radial dial after I summoned demons so that demons keep following me and do not disappear after resting.
Never having used the spell let me ask- does it have a set duration? Maybe just resting long enough will make it go away?