No, I think you'll be stuck with it for 8 weeks. See the article at Reaper’s Rewards Event | EA Forums - 4967458. Line #7 in the numbered list says,
The Reaper’s Rewards Event will have six weeks of Quests and two extra weeks at the end, so there’s plenty of time to catch up and complete it!
I think the best thing to do is complete the challenges if you don't want to look at them. The challenges get removed from your screen once they are completed and this week's challenges were pretty simple. Of course, you'll still have the icon for the event but that's easier to ignore than a task list.
I don't know why there isn't an opt-out but my guess is that it's probably because the developers didn't expect many people who wouldn't want to get free stuff.
But I, like many I'm supposing, are in the middle of a storyline. I write about what happens with my Sims. This is a total distraction. When I first clicked on the page, that starts this whole mess, I had no clue as to what I was getting myself into. I'm boiling mad at the moment. I don't like to be interrupted when I'm working on a story.
