To reformat the USB drive, plug it in to your Mac and open the Disk Utility. Once open, find your USB drive and click on the drive itself (not a partition). Then click "Erase".
Here you can choose a name, format, and scheme.
I'm going to name mine "Ubuntu image" but the name doesn't really matter. Then we'll to use the format MS-DOS (FAT).
When you're ready, click "Erase"
Using the df command we're going to determine the mount point.
Under the Mounted on header look for the name of our USB drive.
So I'm looking at this line:
/dev/disk2s2 15Gi 1.0Mi 15Gi 1% 0 0 100% /Volumes/UBUNTU
Take note of the filesystem name in the first column. Mine is /dev/disk2s2. disk2s2 is actually the partition name, but what I'm really concerned with is the device name, which is disk2. Copy this device name.
Open Disk Utility and find the partition that you created in the left column. Now click the eject symbol.
Locate your disk image (it's likely in your Downloads folder).
Now we're going to need the device name and disk image name, and run the following command:
sudo dd bs=1m if=DISK_IMAGE_NAME of=DEVICE_NAME
Mine looks like this:
sudo dd bs=1m if=ubuntu-16.04-desktop-amd64.iso of=/dev/disk2
Once this command completes, you'll have your bootable USB drive.