How about an equipment spell called Amulet (or whatever) of Dismissal? It would let a mage remove one conjuration spell that he created from the game. For example, if a mage were to be teleported into his own poison gas cloud, he might be able to quick cast a dismissal action to remove the conjuration so its effects are not felt during upkeep.
As an enchantment with a similar ability, dismissing his own wall with the passage attacks trait would be good, in which case he could reveal the (one use only?) enchantment to dismiss the barrier. The same might be done for any wall cast by the mage he were about to be slammed against.
On the question regarding whether conjurations should be allowed as targets, however, someone could make an argument that the "original" casting magic has taken effect and that a physical object remains. That is, if we presume that they are physical manifestations brought into being at the time of casting and have no on-going magical effect, then there would be no magic effect to cancel. However, the mage knew how to create the object with magic, he might also know how to undo it with magic.
Now that I think about it... this might be an interesting mage special ability for a "conjurer" type mage. No equipment or enchantment needed... just leave it as a quick spell ability that he is able to be cast reflexively (counter spell step, or whatever it's called).
