In security, the answer to any question which contains the phrase "100%" is always a big fat NO.
Simply formatting, overwriting, erasing, or whatever else you can come up with, is not enough. Why? Because in all of these cases, you always have to go through the stick in order to do that. But, if I am an evil USB stick, and you tell me to erase myself … why would I comply? I could simply pretend to be busy for a while and then tell you "I am done", without ever having actually done anything.
So, for example, the stick could simply ignore all write commands. Or, it could perform the write commands on a scratch flash chip, wait for you to verify that the write really did erase everything, then swap in the real flash chip. The USB stick could contain a USB hub and actually be two drives, one of which only gets inserted very briefly while you are erasing the other one (which takes a long time, and thus it stands to reason that you are going to leave your computer and grab a coffee or something like that, so that you have no chance to notice).
Also, the USB drive might not even be a USB drive at all. It could be a USB keyboard which extremely quickly types some commands into your computer. Most operating systems do not verify the identity of attached keyboards. (Yes, this attack does actually exist in the real world.)
Or, it could be a USB 3G modem … and boom, your computer is connected to an open unsecure network again.
It could possibly not even be a USB device. It might be a microphone or a camera, and simply use the USB port for power.
Or, it might not be trying to install malware on your computer, but simply aim to destroy it, e.g. by putting 200V on the data lines.