How to stop reindexing in Elasticsearch?

This is the one that worked for me.

POST _tasks/_cancel?nodes=node_id&actions=*reindex

You can use the Cancel Task API if the Task is cancellable

Quoting from documentation

Any Reindex can be canceled using the Task Cancel API:

POST _tasks/node_id:task_id/_cancel

The task_id can be found using the Tasks API.

Cancelation should happen quickly but might take a few seconds. The Tasks API will continue to list the task until it wakes to cancel itself.

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Using the command below, get the task ID and follow

GET _tasks?detailed=true&actions=*reindex

Use the id given out from the command and cancel using the Task API

 "tasks" : {
    "r1A2WoRbTwKZ516z6NEs5A:36619" : {
      "node" : "r1A2WoRbTwKZ516z6NEs5A",
      "id" : 36619,
      "type" : "transport",
      "action" : "indices:data/write/reindex",
      .....
}