What to do with Level-80 Boost?
You can use it on a L80 character and reap the rewards.
(Official video)
Good luck.
You can’t destroy it but you can use it on a level 80 character and it will give you the items but won’t change anything else. You get some pretty nice stuff that way: https://wiki.guildwars2.com/wiki/Level-80_Boost_Package
Alternatively you might want to hold onto it just in case. I don’t like using boosts either, I never even use Tomes of Knowledge on my permanent characters (only on ones I make for a specific short-term purpose like replaying a story chapter or getting a black lion key). But I decided to use this to level up a Warrior because I was working on getting all 9 professions to level 80 and that’s the one I found most boring (and was therefore least likely to play enough to level up).
“Life’s a journey, not a destination.”
I used it at 79.5 for the free armor set. Then decided I didn’t like guard anyway, but that is another story.
Just burn it there – or sit on it until you want an alt for something and want to skip the old world again.
PS: or just destroy it. You are not obliged to use it.