Hello Vincent,
So essentially you are asking how to delete the workflow job instances, not Identity Portal requests. Identity Portal does not store the workflow instances. They are stored in the workflow database.
You can delete these workflow instances by connecting to the Workflow Designer GUI.
Go to Basic Tasks, Open Job, select the workflow process you have configured for your workflows, for example TwoStageApprovalProcess and then select and delete or suspend the job instances you want to eliminate. Active State means the workflow still has not completed.