For Default Service driven tasks.
You cannot Turn off Tasks directly but task are created from Services being turned on for Clients and Dates being entered. If you do not want tasks showing for Clients then you would need to Turn the Services off within the Clients File, or remove the dates so not task is generated.
If you are wishing to Turn off Preparation Tasks this can be done by using the workflow Feature.
Go to Tasks > Workflow Manager > Settings.
Look for a column labeled Enabled and use the toggle switches to turn off preparation tasks.
Click Review Changes, then toggle on Update Existing Tasks to remove current tasks from all users.
Finally, click Update Workflow Settings to save your changes.
This should help clear up your interface by removing the task indicators.