Once submitted to JET, you cannot delete products from JET system. You can only archive them, which is equivalent to deleting. You can bring your archived products back in the future. However, archiving and bringing products back is usually a slow process, if you want to take your product down from JET marketplace the simplest way is to put qty 0 for a selected item.
Please note that archiving products on JET is somehow buggy and it takes several days to be recognized by JET API. Very often merchants need to archive products multiple times before JET API sees this. If you experience an issue with archiving your products, please contact GeekSeller support for assistance.