You can disable a Synchronize account without removing the calendar data associated with it. You can use any of the following techniques depending on your needs:

• Clear or change the login_name to prevent anyone from logging into that account.
• Change the Synchronize_alias to something (e.g., "disabled account") that will dissuade others from scheduling this account.
• Prevent others from scheduling this account through restrictive settings in the Options -> Calendar Permissions dialog.