First of all I plug in USB stick and run dmesg to find the attached device, for example /dev/da0.
Then I check current partition information with gpart show /dev/da0.
I use dd(1) to fill the device with a bunch of zeroes.
dd if=/dev/zero of=/dev/da0 bs=1M status=progress
reuse
To make that stick usable again it has to have a partitioning scheme and a filesystem.
Here is an example for GPT with UFS.
gpart create -s gpt /dev/da0
gpart add -t freebsd-ufs /dev/da0
newfs /dev/da0p1
Ready to be used.
mount /dev/da0p1 /media/usb
# do stuff
umount /media/usb
throw away
If I want to throw away that thing I also encrypt it.
Initialize GELI on the whole USB disk.
geli init /dev/da0
Destroy the encryption key.
geli clear /dev/da0