Hi Pritam, If you can regocnize them on the fact (in a field) that they left the company you can make a view in which they all show and then use "change all" to check the switched off checkbox. After that they aren't able to log in anymore. Hope this helps, Lex.
You can also use the SD API with the setDisabled method on the IAccount object to write a java servlet that would loop thru an array of accounts and block each one.
Or if you don't want to go to that trouble, you can write a SQL statement that sets the value of REP_ACCOUNTS.ACC_DISABLED = 1 for each of the accounts you want to block, but HP frowns on using SQL directly against the db. I had to write a script to block accounts that had a last PW modification date over 120 days old in order to satisfy our security folks.