Good question. Tasks can only be deleted one by one I'm afraid. It is a good input for future improvement.
We do have some bulk delete functionality but this works for app items (if you are using a task app), not the regular task function. Learn about the bulk delete function here: https://help.podio.com/hc/en-us/articles/201019808-Bulk-delete
//Sara - Podio
Sara - Podio