Based on the idea of a memento:
* Forgetfulness: This is more of a concept than a specific word, but it conveys the opposite of remembering something through an object.
* Disregard: This implies a lack of interest or attention, the opposite of cherishing a souvenir.
Based on the idea of a keepsake:
* Discard: To throw away, the opposite of keeping a souvenir.
* Give away: The opposite of keeping a souvenir for oneself.
Based on the idea of something bought as a reminder of a trip:
* Local product: This could be something bought in the place you visited, but not specifically as a reminder of the trip.
* Non-touristy item: This implies an item not specifically designed for tourists.
The best antonym will depend on the specific meaning you want to convey.