You can trick or treat twice in 60 minutes, you need to be pretty fast though.
To do this go to an inn-keeper and zoom in to first person view, position the chat-box right next to the inn-keeper, hover your pointer over trick or treat and position it to one of the sides (the side must be sided with the inn-keeper).
Now click trick or treat move your mouse to the inn-keeper very fast and click again, you will be able to select trick or treat again, even though you are allready debuffed.
Don't say it doesn't works it just takes some practice
You can exploit stuff like this for other events too, an example would be the christmas event, you must kiss npcs under a misletoe, just make a macro wil lots of /kiss, and spam it on the npc, you will get loots of goodies, and a lifetime stock of candy canes if you wish