The above is a picture of of a 2 penny iron finish nail and if I had my wish every Lionfish in the Atlantic basin would receive one (or two) for Christmas.
Why an iron nail? It's iron so a miss is no harm to the reef, a hit and you have a stessed Lionfish and a stressed fish on the reef is soon to be an ex-fish on the reef.
I have been thinking about this problem and this is an engineer's solution. I see a semi autonomous ROV searching, searching then using optical recognition it see's a Lionfish, then a human gives the command to shoot one or two of the above. Then all depths and ares not accessable to divers could be swept of this scourge.
That's long term, short term is to develop the Lionfish nail gun. I see this as a modified paint ball gun using CO2 or air. Why not a nailgun, that is design to hammer a nail with a piston vs. shooting out a barrel. This should also be designed with an absolute safety such that it could not be fired out of the water. It would be very short ranged, a meter max, I would guess.
Now saying all that, I must also add, I have no way, time or means to do any of this and I'm just throwing it out there hoping a Bill Gates or Dean Kamen will hire me for a gazillion dollars a year to make it happen.