Hi,
Is there a way to decrease XP points?

With mods....

Which mod?

If you're playing on PC, you're in luck. Just search the http://newvegasnexus.com/ for an XP- decreasing mod. I'm sure someone's made one. And maybe you can use he G.E.C.K to modify XP rates.

Mind if I ask, why does one need to decrease XP?

If you plan on playing through just about every side quest, it can be a bummer to halt all character advancement only about halfway through the game.

My advice if you can't use mods:

Take the Animal Friend perk to avoid XP from attacking hostile animals.
Never sleep in a bed marked "My Bed." That way you at least avoid the 10% XP bonus from being well rested.

Thanks, coinop25!