cannot defeat auto-correct spelling changes

CorelDRAW keeps changing my capitalization of technical abbreviations in a way which makes me look stupid: "MHz" becomes "MHZ", and "GHz" becomes "Ghz". I have tried adding the correct replacements to word lists, to no avail. I can't find a way to disable this auto-correct feature entirely, which would be my preference. Has anyone fought auto-correct, and won?