The law follows a series of stings which found people clocking-in to work before leaving to engage in more appealing pursuits.
“For those who get caught clocking-in and then leaving, the ride is over,” Renzi said.
“Those who do so are cheating the state and will be sent home.”
Employers who turn a blind eye to time-stamp swindlers also "risk being sent home too", Renzi said earlier this year as he announced the crackdown.
The problem of time clock dodgers hit the front pages in October last year after 35 people were arrested in San Remo in northwest Italy and 195 people were placed under investigation for absenteeism.
Traffic cop Alberto Muraglia, 53, who lives in the same building he works in, was secretly filmed by financial police as he clocked in in his underwear before apparently going back to bed. His wife was also filmed clocking in some days on his behalf, in her nightie.
Others were caught on camera clocking in and then going canoeing, shopping or out with friends.
In January, nine staff at a museum in Rome were suspended after being caught swindling the time management system. In the same month, staff at a council in Sicily were exposed for clocking in, before spending the day pursuing more appealing activities such as "shopping and hunting".