I'm not sure how you "messed up a quest" but you shouldn't ever have to delete a character to solve that. At worst you might have to restart the quest itself, but I'm not sure what would make you need to do that either.

The simplest way to not get the buff is to not pick a preferred server.

There are benefits to a preferred server though (IIRC free feather tokens for buying gear, and 15 days' free play time for reaching level 30), so a good way of managing the buff is to pick up a second class and switch between them. EXP for each class is earned separately and only goes to the class that you are set to when you complete the quest, so you can switch between them and do a lot of your quests on that second class to level it up instead of your main. You can even pick a crafting or gathering class for this, or a battle class you don't really want to play.