In the rules, it says:
All units except for the God can leave the battlefield. The outside of the battlefield is considered to be an accessible area. If a unit leaves the battlefield, it leaves the game for good. If it was carrying an omphalos, it is removed from the board."
Does that mean you can mighty throw an unit out of the board to kill it? Monster, hero and troop included.
P.s.: this is in the 3v1 mode
We are still playtesting the 3 vs 1 mode. It was added lately because a significant number of backers asked for it. It may be changed in the final game and I think that in the end the sentence will be removed from the rules. To be confirmed.
Please note that all of the other modes (1vs1, 1vs1vs1, 2vs2 and 1vs1vs1vs1 are final and heavily playtested.
Anyway if you do not consider the 3vs1 mode (against this one is not finalized), the general rule is that no unit can leave the game after having entered it except from when it is killed. The limits of the board are considered as impassible terrain. You cannot throw an opponent out of the board.
Please note that a scenario may include special rules which could allow to throw enemies out of the board. Could be fun.