1)Go to Tasks tab and heighlight your task or query the task.Click on the Edit tab in the bottom pane.Select Inactive checkbox for that task and save it.
2)Check if that task has been assigned with any Parent Group(Task Group).If so you might disable Parent task as well by going to task groups and selecting inactive in the bottom pane of the highlighted tasks. Save it.
3)Assemble your subject area and then build your EP.
Run full load using the EP which you created.
To verify that tasks has been removed you go to Execute -->Current Run-->Tasks -->Query that task ans see it is not getting listed.
However I strongly disagree to disbale any tasks without debuggin it as it may cause data issue in many of the tables.
Think what is best suited for your environment and situation.