You can turn preparation tasks off using the Workflow feature.

Navigate to Tasks > Workflow Manager > Settings, here you'll see a column called Enabled. You can use the toggle switches in this column to to toggle off preparation tasks, then just click Review Changes. You can then toggle on Update Existing Tasks to remove the existing tasks from your list for all users, then click Update Workflow Settings to save the change. 

To switch on the workflow feature navigate to Settings > Account Settings > Workflow tick on Enable Workflow and click Save Settings.