Killing the Q Mermaid undoes everything.
The Q Mermaid is possibly the one that started it all, so if she ever gets killed, all the spells cast through her curse are undone, and those that became mermaids are restored to humanity. Of course, that does mean there are probably many divorces in the future. Alternately, killing the Q Mermaid makes you the new Q Mermaid.
'Q' is probably short for 'Queen', after all, and the mermaids need a leader to make the rules.