Please add a setting to opt out of XP bonus if there isn’t one already and/or an option to store the bonus and apply when needed on the current or another character.
Just to consolidate all answers into an easily read post (and add a couple of hints that were not explicitly mentioned):
People, when leveling up and playing on groups, generally run quests with a base level 2 levels lower than their current one. That's because when you run a quest on elite or reaper it gains two effective levels and this way you can still run them "at level" and without any xp penalty.
Also, as others mentioned, you do not need to take a level as soon as you get enough xp, and any extra xp will continue to be accrued until you have enough to get to the second next level - 1xp. This happens because you cannot accumulate 2 level ups at once (while doing regular leveling with regular xp). At level 10 you can get enough xp to reach level 12 - 1xp.
If you don't care about the extra xp after the hard cap and since there are enough quests at each level to keep leveling up at least until you reach the next one anyways, then there's literaly no need to "turn the xp gain off" - you already can effectively do that by just keeping running quests without leveling up.
So if you want to keep running all quests before you outlevel them on a difficulty scale (since xp won't be an issue here), one thing you can do is:
Don't take level 4 until you finish all base level 1 quests, then don't take level 5 until you finish all base level 2 quests, and so on, always keeping those 2 levels as a buffer, if running quests at elite or reaper.
If you are soloing, however, then you can of course adjust this buffer to any number you see fit. Nothing prevents you from choosing how much xp to bank or how many levels to take and whether to do the quests while under or overleveled. Just have in mind that you won't be able to run a quest on reaper difficulty if you're over 4 levels above it on heroic quests and 6 levels on epic ones.
Cheers,
NH