If you're on Lion or above go to the Go menu in the Finder. Hold down the Option key and Library will appear in the menu, click on it. Go to the Spelling folder, Control-Click on Local Dictionary, choose Open With/TextEdit. Delete the words you don't want, delete blank lines if any. Do a Save. If it says the Dictionary was changed etc click Save anyway. Reboot the computer for the changes to take hold.

If you're running Snow Leopard or lower the Library folder will be in Macintosh HD/Users/you.