With the 25R3 release, users can now cancel running Workbench Reports. After you have configured this feature, see Run, Download & Copy Workbench Reports and Generate & Run Reports from Workbench Report Sets for more information.
Configure Cancel Report Actions on Workbench Reports
To configure the Cancel Report and Cancel Reports actions, assign the actions to the Workbench Report object and add the actions as user actions to the applicable states of the Workbench Report Lifecycle object lifecycle.