As of today you may harvest a lionfish if you are in possession of a valid saltwater fishing license. The NOAA has called them an invasive species so there is not limit. The term harvest means the ‘reduction of a resource’. For present, if you kill one and leave it on the bottom, you still have caused a reduction and must hold the proper license. A valid saltwater fishing license if available to both residents and non-residents and there also are exceptions for various people like children, seniors which are too numerous for this post and should be researched separately. It is nominal fee and if you have a lobster license, you are covered.
Ways to kill them:
Some use a net and snip the poisonous spines off with scissors
Spears – a tri-tip or paralyzer tip to catch the fish. This tri-tip ensures the catch stays on the spear until you can secure it.
If you do not want to bring the fish back to the surface, there are lionfish killers (e.g., the “Lion Tamer”) that are designed to poke a hole into the fish to kill it but allows you to easily discard or remove the fish from the spear.
Now that you have him, you should use cutting shears and remove the spines across their backs, pits, chest fins, and anal fins. You do not want to get poked with one of their spines because they are poisonous and will cause a painful sting. To be on the safer side, wear non-puncture gloves. In the event that you are stung, place the wound in or under water as hot as you can stand to relieve the pain.