If "memento" refers to a reminder of a past event:
* Forgetting: This is the opposite of remembering, which is what a memento does.
* Oblivion: This implies complete forgetting.
* Amnesia: This refers to a loss of memory.
If "memento" refers to a physical object:
* Disposal: This refers to getting rid of something.
* Destruction: This implies breaking or erasing an object.
* Discard: This means to throw something away.
In the context of a gift:
* Gift: This could be a more neutral opposite, but in this case, a gift is something you receive, while a memento is something you keep.
* Giving: This is the opposite of receiving, which is how you acquire a memento.
The best antonym for "memento" will depend on the specific context in which it is used.