You can delete tasks that you have delete rights to from the app association page.
Do the following:
Select Apps on the QMC start page or from the StartS drop-down menu to display the overview.
Select the apps that you want to delete tasks from and click Edit in the action bar.
Select Tasks under Associated items.
The App association items page with the Reload tasks overview is displayed.
Select the tasks to delete and click Delete in the action bar.
A Delete dialog is displayed.