Supercan allows you to easily delete files in the Trash. The secured shredding overwrites files with random data, making it far more complex to recover data that could be confidential. It takes longer, but it can be applied file by file, not on the whole trash.
Supercan displays the trash of each volume connected to the computer, and each one can be emptied. That is, at last, a way to make room on a USB key.
Supercan requires at least Mac OS 10.6, which makes it compatible with most machines.