Unfortunately you cannot do this out of the box with Magento.
The permissions are not this specific. Any user that can add or edit will also be able to delete.
This functionality could be accomplished by creating a custom module though.
Hope this helps.
EDIT
The quickest and easiest way to do this would be to create a small module which adds the current logged in users role as a class to the page body tag.
Then you can use css to hide the delete button when anyone logs in who has that role. e.f if the role was "editor"
body.editor .adminhtml-catalog-product-edit button.delete { display: none; }