If people are taking their "earned" leaves within the defined limits, then there should be no problem with that. Are you planning to reduce the "1.5 leaves a month" rule itself? If productivity is the only issue, then there are other ways to resolve it. You (supervisors) can give them target-based work. For example, by the month-end, they have to deliver work worth 160 hours. If they can manage the work by working late or on weekends sometimes, then it should be acceptable. You can also use the leave retention/encashment policy so people can avoid taking leaves just because otherwise, they would lapse.
You should definitely use some discretion for granting leaves, even request people to change their leave dates when two people's leave is coinciding and work is affected. But, if you do a little research, you will surely find more suitable ways of increasing productivity without restricting the earned benefits of employees.
The growing phase of a company can only be sustained by the dedication of employees. But if you make the rules very strict, so that restricts the basic rights of employees, then you cannot expect dedication from them.
Best Regards,
Amod Bobade.