The moment a candidate accepts the terms and conditions of appointment, issued in your company's letterhead, by way of signing and returning the duplicate copy, it is legal and binding on both the parties. As my learned colleagues have mentioned above, you need to look within to change the policies relating to recruitment, employment etc. There is an old saying " When you pay peanuts, you get only monkeys". Therefore, find why your employees leave abruptly, how is your compensation structure when compared with your competitors, where your employees are joining, whether or not improve the working conditions etc. Further, please remember that in no organisation, employees join and retire from the same company. These days, opportunities are abundant and people keep moving across the organisation, cities, countries. Nobody is indispensable but at the same time, too much of exodus beyond 15 to 20% per annum, indicate that there is something wrong with the organisation.
So do some introspection first.
Best wosjes