Hi Rabi,
The reason for maintaining ESI Masters is that - Earlier the employee was in ESI bracket as his gross was less then 15Ks, But now he received a salary hike which added up and made the gross more the 15Ks. Earlier We were to deduct his ESI and We deducted but now We dont want to because now he stands out of ESI bracket.
Am ok with Suresh's solution (and that is what I have done for earlier employees) but I want to know if there a solution to design this scenario when if the employee goes above 15K the ESI should automatically stop and I need not to delimit/delete his IT588 
Thanks,
Harriss.