Go to the Content Library \ Products view. If the product has a “u” then its a “user defined product” and you can right click to delete.
If it does not have the “u” then it is a system product and cannot be deleted.
If you are sure you created the product, then you are right but DAZ Studio brilliantly changes all user defined products to system products every time you import a new product. In other words - you have only one chance to delete a product you don’t want.
Of course there is a workaround - simply reset your entire database, reimport all your metadata and reorganize all your content by hand. This shouldn’t take more than just 4 months for around 60 GB of content