The Installed Dictionary list in the Spelling panel of the Options dialog box shows the dictionaries currently available for spell checking a form. You can delete one or more of the custom dictionaries you no longer need. When you delete a custom dictionary, it is removed from the list of installed dictionaries. The actual dictionary file (*.clam) is not removed from your computer’s file system.
Note:
You cannot remove My Custom Dictionary because LiveCycle Designer ES always refers to this dictionary whenever you spell check a form.
See also 

To remove a dictionary