Place abilities can only be negated in the same phase they are used. If you place Abom in my territory, I can use Covenant of Noah to negate the placement during the phase it is played, so that it is discarded at the end of battle, or more likely immediately if you used it with High Places.
If however, you don't negate the placement when it is placed, it sticks. You can use CoN to negate it for one phase (i.e., if you negate it during battle, you can draw as much as you want with no penalty) but it remains in your territory to affect everything after that phase.
