Killing a Jellyfish is actually very hard because they are made up of many smaller organisms called ‘polyps’ and the Jellyfish itself is technically a colony of these smaller organisms all cooperating together. Simply hacking at a Jellyfish will likely lead to little more than an inconvenience for these amazing creatures, and as they live in the water there are only a few sure ways to kill one completely.
This would be to remove them from the water and allow them to die on land by drying out, killing all of the organisms at once and together to prevent healing and regeneration. This is often how Jellyfish are found, washed up on beaches slowly dying from exposure to the heat of the surface. To be totally sure that it is dead, some say that burning the carcass or using acid to break down its cells are the best way to be sure that a Jellyfish is totally dead.
The truth is, killing Jellyfish is a very difficult task because of their biology and the environment they inhabit. In the water, it’s very difficult for a single person to hurt Jellyfish because they can swim quite effectively and water doesn’t allow us to generate much force or coordinate an effective method of harming these creatures.
In fact, the most common way humans harm Jellyfish is totally by accident.
Jellyfish are often caught by fishing vessels as bycatch and are often killed in this way. They also tend to watch up on beaches occasionally, becoming stranded and dying due to being outside of the environment they are so specialized for.