plug it's nose ah-ha-ha-ha-ha. 1-2 teaspoons of baking soda in a quart of water. Mix in a bottle and use this water to refresh your tree throughout the season.
The water in your tree stand shouldn't smell. You should be putting fresh water in your stand daily and the tree should be soaking it up daily. With a fresh cut tree I have had to fill the stand several times daily but it does slow down the longer the tree is up. But if you are concerned you could try removing the excess water with a turkey baster or a small shop vac and put fresh water in daily. Some people use 7 Up pop/soda rather than water.
Many don't get to the tree daily but thank you.
Why is it smelling? It shouldn't. Any pets in the house? Was a fresh cut done just before placing in stand? Check every day. Add a bit of baking soda to the water and see if that does the trick.
Place a open box of baking soda near it and that will absorb the scent.
A drop off of bleach will kill the odor causing bacteria