NetSuite does not currently provide a direct option to stop a running Map/Reduce script by design.

However, if a user needs to stop a stuck or long running Map/Reduce Script, a simple trick can cause its execution to forcefully halt. That trick is to undeploy the script via its Script Deployment. Refer to the following steps to do so:

Uncheck the Deployed checkbox field in the Script Deployment of the Executing Map/Reduce Script.

Navigate to Customization > Scripting > Map/Reduce Script Status
Click the Deployment ID of the Map/Reduce Script to stop
Click the Edit Button
Uncheck the "DEPLOYED" checkbox field
Click Save
Important:

Forcing a Map/Reduce Script to stop might cause record state inconsistencies if the script stops in the middle of an operation