Many developers wish to remove the "Add to Dictionary" button, normally this comes from a lack of understanding.

When a user adds to dictionary, the word is stored in a cookie on their computer - not on the server. No central dictionary is updated, so there is no harm in keeping the button.

However ASPSpellCheck allows you to remove the "Add to Dictionary" button using the ExternalCSS property to disable or hide it.

However......... 

The [Add to Dictionary] button can now be wired up to add to the Custom Dictionary as an option: http://www.aspspellcheck.com/aspspell-manual-The-Settings-File.asp

In the Settings File (Settings.inc)

setting_CentralDictionary - if set True, words 'added' to the dictionary are written into the custom dictionary - "custom.txt" . All users will then have access to these words.

This is useful in intranet applications, but less so for public websites. If you wish to use the custom dictionary, you must assign IIS (IUSR_MACHINENAME) write permissions to the /ASPSpellCheck/Dictionaries directory.