As rightly said above, people negotiate with the salary and make use of the appt letter to quote their amount outside. This has become a common mindset of people. Firstly, they shud not join if they do not agree with the salary. But in a haste they join sum company and later quit when they find a new job with higher pay.
What can be done is one can explain the positives of being with the company even though the salary is less and show them their career path. Conduct the induction properly and make them feel wanted in the company even if they have just begun. Do not issue the appointment letter quickly. Communicate to them that y u r not giving them the letter and let them know the probs u have faced after giving others the offer letter.
You cannot make delay in giving them the letter, but once they have joined, give them in the profile what they expected. If they need to learn sum more to reach to that stage, explain it to them accordingly.
You can try to do everything from your end.... but if they still leave.... u cannot do anything..
then just thank them that they left atleast within a month and dint work for so many days that they had to be given a salary.
Regards,
Ashlesha