How do you completely delete files from a USB Flash Drive?

I keep deleting things from my usb flash drive, but it doesn't actually delete, if that makes sense.

IE: If I have a 1gb memory card, and put a 500mb file on it, then delete it, it still says I only have 500mb available.

Any ideas?

Empty Trash with the USB-stick attached.

Same as Windows operations. Drag content/s to trash and as roger advised, empty the trash with the stick still connected. Then drag the stick icon to the trash to disconnect.

the easiest way is to move it permanently off the stick first by clicking and pressing cmd...

then you can delete it whenever u want