I explained to the users of my SharePoint to upload a document to the library and start the Approval workflow. All was going well until one user went to the tasks list, created a new task and attached the documents to it. This is definitely not what I explained and I have no idea how he even knew to do this. This cannot continue as the documents must be easily visible in the document library. How do I stop users from adding new tasks to the task list from the list itself, without stopping them starting workflows (and the tasks being created) from documents in the library?
You will need to create a new permission level, I would copy the Contribute level and remove the Add Item permission. See this link for managing permission levels
Then change the Task list permissions so users that use the workflow do not have Contribute permission but have the new permission created above. You will need to break permission inheritance on the Task list for this.
The workflow should still run fine and create tasks for the users but the users will not be able to directly add tasks to the task list.