Remove all traces of GPT disk label

I've got an USB pen drive and I'd like to turn it into a bootable MBR device. However, at some point in its history, that device had a GPT on it, and I can't seem to get rid of that. Even after I ran mklabel dos in parted, grub-install still complains about Attempting to install GRUB to a disk with multiple partition labels. This is not supported yet.. I don't want to preserve any data. I only want to clear all traces of the previous GTP, preferably using some mechanism which works faster than a dd if=/dev/zero of=… to zero out the whole drive. I'd prefer a termina-based (command line or curses) approach, but some common and free graphical tool would be fine as well.