Agreed with Ms. Ankita's comments. I would like to tell you one thing: in today's era, you cannot stop any employee from leaving the organization, and you should not stop anybody's salary, as that is what the job is all about; I mean, everybody is earning for bread and butter.
Organizational Operations and Recruitment
Firstly, every organization has a way of operations, and hence recruitments from organization to organization vary. Furthermore, it should be very clear what the job descriptions are and what they have to perform.
Training Process in BPOs
I think since it's a BPO organization, the generally selected candidates would have to undergo training, and only those who have completed the training are then put on the desk, if I am not wrong.
Recommendations for Recruitment and Training
I would suggest here that the recruitment and training process should be so rigorous that it will automatically help you judge the candidates' nature. Ensure that those you have in the final round are the cream of the candidates who are really interested in making their career. Then, the percentage of candidates leaving would be very minor.