How do you delete a difficult field research task

I find that the ones I want to dump are the ones that will give me poke balls I have too many of, or two raspberries, or one pinap berry. I'd rather do something harder for a reasonable encounter or something good like Ultra Balls or Rare Candy.

Also note that if you dump a task, and then change your mind, you can go back to the same pokestop and spin it again to get the same task. For the few times I've done it, I've also gotten exactly the same reward. Suspect this always happens, don't have enough evidence to verify it.