You can disable spell checking by opening <ZD data root>/profile/user.
js and adding:
user_pref("layout.spellcheckDefault", 0);
<ZD data root> location depends on your platform:
Mac: ~/Library/Zimbra Desktop
Linux: ~/zdesktop
Windows XP: C:\Documents and Settings\<user>\Local Settings\Application Data\Zimbra\Zimbra Desktop
Windows 7/Vista: C:\Users\<user>\AppData\Local\Zimbra\Zimbra Desktop