Gotcha. In that circumstance, what occurred is that you just smeared books with mouse-bait. That ruined them even before the mouse got to them, so the point is somewhat moot.
However: that point by itself means that the potential for damage after Wizard's War is ongoing. You need to undo your actions so that NO FURTHER DAMAGE occurs. This is why using summons (such as your example with the Hunter, above) or with the mouse are questionable activities: you can't control them, and in particular you can't easily turn them off when the War is over.
What it sounds like you're saying is "this sort of damage isn't easy to turn off! Therefore it's OK to use it in Wizard's War!" My response is, "No: that's why it's NOT OK to use such spells in Wizard's War - and using such uncontrollable spells is a good way to get punished after the fact by the Tribunal."
In this case? Make a reasonable attempt to inform his sodales that you smeared cream cheese all over his library. They may not believe you, or they may not be willing to let you enter their friend's sanctum. But by making a reasonable attempt to undo your actions, you can say you made a reasonable attempt prevent any FURTHER DAMAGE. (You could likely get a Gernicus to accompany you into the Sanctum, to ensure that you only removed your Cream Cheese bait, and nothing more.) Alternately, accept the fact that you may be punished for your actions, and don't use long-term, uncontrollable effects in Wizard's War again.
A similar effect would be "use Perdo Terram to hollow out the area underneath the magus' sanctum, so that in a season or two it collapses." Ignoring the fact that such an attack is an AoE effect, and thus would damage the Covenant (rather than just the magus' stuff), you had a reasonable expectation that the FURTHER DAMAGE would occur after the War was over. Thus, you have a responsibility to fill in the dirt (with a Creo Terram ritual) before anything untoward occurs. Or offer to hire someone to do it and provide the vis, or something like that.
My take on it: If the poison is still in his system, continuing to do damage? You're still liable. Conclusion: don't use poisons that stay in someone's system, continuing to damage them for weeks on end. It's a bad idea for Wizard's War, as there's too much (temporal) splash damage. If you use a poison, make sure it's a fast-acting one that kills them (or does whatever) within the confines of the month.
If the poison is flushed out of their system, but the damage is done, and they end up dying of renal failure? You're not liable. So you don't need to clean up your damage - but you do need to clean up sources of FURTHER DAMAGE.
EDIT - this would be the difference between a poison that does damage-over-time (ie, "a light wound every morning for 3 weeks"), and builds up damage, vs. one that hits you once ("Causes a heavy wound once"), but kills you when you fail to make the recovery roll.
Wizard's War is conscribed by space (you can only attack your target, or his stuff) and time (only within 1 month). All the examples of Wizard's War spells (such as Intangible Assassin) are all spells that have short duration and target only a single magus. There are no area effects, and there are no delayed effects. Personally, I take this as evidence that AoE and Time delay effects are a bad idea in Wizard's War, due to the fact that they break the limitations of time and space imposed by the Code.