They are beautiful but twice deadly.
The crown of thorns eat off the living part of the coral, only the dead lime skeleton remains.
You can not stab them, because at the moment of death they fire hundreds of seeds from which grow new crowns of thorns.
Catching them is also difficult because these thorns are extremely poisonous. A small prick and a purulent abscess is guaranteed for weeks. If you step on it, you will be in the hospital for months or years, if you survive.
The only way to remove crowns of thorns is to carefully place them in a box underwater and bring them ashore.
A job for diving pros.