Some task are provided by standard. You can hide any tasks in any POWL Inbox type using the customizing table IBO_C_WF_TA_P_HC.
You can do this by the corresponding IMG-node:
SAP Implementation Guide -> Cross-Application Components -> Inbox -> Assign Task IDs to POWL Types. (This IMG task has a detailed task description that should help you to understand the needed procedure),
In this task, select "Hide Task IDs".
Now, you can enter all task IDs that should be hidden in your application.
The standard tasks can be found in table IBO_C_WF_TA_P_TY.