Can there be a Gmail Gadget App that permanently blocks or deletes spam?
If cluttering your Trash is not desirable, you could filter those messages to a temporary label instead, then use an Apps Script to periodically delete "old" filtered spam from that label. That would ensure that the top of your Trash is clutter-free. If the volume of spam you receive is limited however, deleting might work just fine.
But I don't know how one would "use an Apps Script" for this purpose.