I faced the same problem few years back. I analyzed it in a different manner. tried to find out why they are sitting for long hours, the reasons i found are astonishing. 2-3 guys are staying back for client interactions, 3 more who are going along with them are staying back, three more to make any changes as per client requirements and few more who are going along with them. Few more are staying back just to take the advantage of coming late and enjoy internet and long chats. you don't believe, the amount of productive work was very low. Then i Discussed the same with my management and scheduled two guys for night shift and asked everyone else to come on time and leave on time. i think it worked out. the company grew to 500 staff now. well. try it, who knows. but first analyze the problem not the employees. solve their issues, deducting their salaries will only increase the problems.
Goodluck
Goodluck