Subtracting XP?

I know that there is a way to add xp by doing:

but I'm looking for a way to remove XP (eg, lower a players level, based on the amount of XP taken away). I've tried doing a negative amount, but it has no effect (it says it 'gave -xx to <player>', but the level of the player never changes.

Any ideas?

 

I would like to sort of append something to this. Recently I did /xp timethor 4500 to see how much xp it would give. Ive tried say 10 in the past and it obviously doesnt give levels, but rather ticks of xp. The command gave me somewhere around 50 xp...but as a survival server admin, i wasnt about to violate my own server rules. So i did /xp timethor -4500....At the time my xp bar went down to no green, but i still had 51 lvls of xp. Thinking it didnt work and couldn't take levels away i went about my business and killed some mobs...no xp was added to that empty bar of 51. Weird. So i went to enchant my items and the enchantment used the proper # of lvls...i eventually used down all levels but now i cant seem to get any more xp no matter how many mobs i kill. This a bug?

Thanks in advance