I'm guessing you're referring to Mac's system-wide spell checker. I haven't figured how to reset it completely, but did discover a way to remove specific words you added.
Open TextEdit, then go to Edit > Spelling > Spelling. In the window that pops up, type in the word you want removed in the text field then click on the Forget button.
I'll look a little longer to see if I find a way to do a total reset.
Edit: OK, found it. In Finder go to username/Library/Spelling and you'll find a file named en or maybe something else reflecting your language. You'll be able to see what words have been added, though it's a continuous string of words with no spaces so hard to read. I tried deleting it, but that didn't make those words "wrong" but I'm thinking it may take a reboot or at least restarting Finder for it to take effect. Let me know if you try.