Since an answer is accepted, I should think that this would STOP any sort of downgrading effort. However it does not. In fact, an accepted answer can continue to go very negative.
I suggest that this bug be corrected....to either prevent downvoting an accepted answer OR preventing the answer from going negative, or set a bottom limit to either -1 or 0.
Furthermore, as author of an accepted message that caused this question, I cannot delete or remove it since it is accepted. But people over time keep adding more negative votes for some reason (probably the piling on situation).