Hi Sarin,
A good question.
Every HRMS software usually are designed to take care of re-hiring process of an employee, which is programmed as per company policy (Check within Software). First, you need to see if you have a "re-hiring" policy in place and if not this is a good time to make one. You will need to get the finance and legal department also involved in this policy framing as more that HR aspect what impacts the employee/company will be employees financials aspect and ofcourse Legal issues related to such re-hire now and tomorrow.
Ideally, to avoid all this trouble usually companies just hires such employee as a new employee, but in the process the employee would lose all his benefits due to non continuity of service. In Management words, this is the cost such employee is paying to break his service for whatever reasons and a deterrent to all emlpoyee going ahead.
In order to protect employees, devise your policy to seperate an employee with acceptable timeframe. It would be a good practice to keep 45 days time period after employee LWD to seperate him from the company system, least he changes his mind. It worked for me and also finance will support due to time they get to arrange funds as well, also retention team get enough time try getting back such employees, if required, post resignation. There is no loss for the employee but the time to get his full and final payment delayed by 45days. I guess its nothing compared to loss, if he re-joins as a new employee.
Ukmitra