You can delete tasks that you created, so they no longer show in the list of tasks to tag your time entries to.
Tap the 3 dots on the top left > Time & Tasks
Tap the Gear icon
Tap the Task you want to delete, then tap Delete
If you want to restore any deleted tasks:
Tap the 3 bars on the top right
Tap on the task you want to restore, then tap Restore Task