In ArcGIS Workforce, it is not possible to delete assignments through the web interface. The project owner can automate and script ArcGIS Workforce to administer projects and assignments, but the workflows are done outside the scope of the web app. A Python script to delete assignments is available on GitHub. Refer to the 'Tip: Delete or archive assignments' section in ArcGIS Workforce: Project management tips for more information.
As a workaround, delete the assignment by deleting the feature representing it on the hosted feature layer in ArcGIS Online or Portal for ArcGIS.
Note: In the Content pane, under Contents, ensure the '<Project_Name> - Assignments' layer is active.