Remove all workflow instances in Alfresco 5.1

Question

I'm purging all unused content on old server Alfresco 5.1.
Now in Alfresco database is about 15 000 completed and active workflows associated with documents.
I would find out how to remove these workflows instances.

I tried to use standard command delete all workflows imeanit in workflow console, but it doesn't work (it runs very slowly and with errors). I can check count of workflow instances in Activity Explorer.

Also I tried to use some ready JS scripts. For example, one of them:

var workflowDefinitions = workflow.getAllDefinitions();
    for (i in workflowDefinitions) {
       var workflowDefinition = workflowDefinitions[i]
       logger.error("Def:" + workflowDefinition.id);
       var workflowInstances = workflowDefinition.activeInstances;
       for (j in workflowInstances) {
          wfInst = workflowInstances[j];
          logger.error("   Inst:" + wfInst.id);
          wfInst['delete']();
       }
    }
}

Unfortanly, it doesn't work for me too!

I know that modifying Alfresco sql db directly is not so good way. However, I have question, is it possible to remove all workflows by deleted required entries (activiti) in sql database?