Let's pretend you had a physical card wall for a moment. You've got some cards that someone thought was a good idea, but on further review, they're not actually so great. What would you do with the cards? Would you create an separate section on your wall for these worthless ideas or would you chuck them in the nearest waste-bin? Yeah, you'd toss 'em, so would I.
So what makes Jira any different? Delete them.
I'd like to muse on what makes Jira different from a physical wall for a moment. The difference between the two is that if you kept the physical cards, they'd be much more noticeably in your way. People have a tendency to want to keep ever half baked idea anyone has every had in an electronic system because it's very cheap and easy to do so. You don't notice how in your way those junk cards are most of the time, but they are. Every time you have to manually filter through garbage trying to find what you really need, you've shot yourself in the foot. Worse, you've shot your team and company in the foot. People are the most expensive part of most projects (and any software project). We shouldn't waste their time by digitally keeping around something we would throw in the trash if it were a physical card.