Hi,
When you click on the "reorder" button, what happens is that the products from the order are added to the cart.
So you would have to actually implement the onBeforeCartSave(&$element, &$do) trigger in order to remove the custom item fields values from $element->products (with an unset).
In the trigger of your plugin, you can easily know if the customer is currently reordering by having a check like that:
if($_REQUEST['task']!='reorder') return;
Because with a trigger like onAfterOrderCreate, it's much more complex as you're not sure that the customer is coming from a reorder, and you have to write MySQL queries to empty the custom item fields of the products of the order.