Welcome @Teodora_Dima
To stop a job that you no longer want to run, you can delete then purge it.
If the job was queued, then it will be removed from the queue.
If the job was already running, it will abort the reporting steps the next time that it communicates with Galaxy and your history.
More details about how jobs execute can be found here → queued-gray-datasets
With more here
Hope this helps to understand how the processing works! 