I think it's silly for players to have the satisfaction of having (for example) 300/300 XP, and then accidentally killing one more monster to level up. Once you get the level you want, just stay there and click the little checkmark that turns off XP.

Although, I think STS should allow the option for one delevel per person. I'm at the 76 cap now, which frankly I hate because it's unbalanced, and I'd love to be deleveled to 56.