Hi,
If you need to delete them for one user, then you can go to SWI5 to get info about them and then to SWIA. Enter search period or task or work item and click Execute.
Then select line and click "Complete Manually" to complete WF.
Another solution (I did this) is to create custom ABAP to cancel all notification tasks using SWWWIHEAD table and SAP_WAPI_ADM_WORKFLOW_CANCEL function module.
If notifications are sent by e-mail, you can not cancel already sent notifications, but you can remove workflow task from notification config.
1. transaction SWNCONFIG
2. Select WORKFLOW line and click on "Filter Basic Data"
3. There should be custom created filters. Select line with filter and choose "Filter settings" and remove your task from list.