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

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