So, my question restated is when you go to Settings -> Accounts & Sync and select the an account that was created that your SyncAdapter is syncing with a c
Another option is to register for the android.accounts.LOGIN_ACCOUNTS_CHANGED broadcast that the AccountManager sends out. Unfortunately, this broadcast is sent out whenever any account is changed and the broadcast does not deliver further information what has changed either.
So you'd have to query the account manager and look how many of "your" accounts it has left and delete the data of the missing ones.