How to remove a word from the spell checker?

How can I remove the word "fuer" from my spell checker? I meant to add the word "für".

I added the word "fuer" to the personal dictionary by mistake. Now I can not undo this mistake because the Ignore button for this word is grayed out.

*Here is what I did*: If you don’t want the word to be treated as an error, click Learn to add it to your personal dictionary. (In some applications, you can Control-click the word and choose Learn Spelling from the shortcut menu.)

*Here is what does not work*: If you accidentally add a misspelled word to the dictionary, type the misspelling in the Spelling and Grammar window and click Ignore, or Control-click the misspelling and choose Unlearn Spelling from the shortcut menu.

Thanks for your help.
Olaf