I find using a simple instructional phrase..."don't slam my trunk" or "don't slam my doors" an effective method...especially when spoken in a stern yet calm tone. And if/when spoken once and it occurs again...additional words are added that include some form of consequences for not following my initial instructions. I've found it to be a great teaching tool for having an appreciation for taking care of things and not destroying them.
There's also the interior trunk/boot release button down in the driver's footwell that can be used to release the trunk: