This is intended behaviour - if an Ldap user is deleted from within Asset Bank then they are kept in the database but flagged as 'hidden'. This is for exactly the reason you describe - so that they are not then reimported the next time a synch happens.
The only way to override this is to run the following statement directly against the database:
UPDATE AssetBankUser SET Hidden=0 WHERE Username='XXX';
Where XXX is the username of the user. Or, if you want to unhide all users, just run:
UPDATE AssetBankUser SET Hidden=0;