A week after a 3-1 romp secured the bragging rights in the Capital Derby, Lazio confirmed their end-of-season purple patch to take a 5-1 lead at the half-time interval.
And Simone Inzaghi's men made sure there was little chance of a comeback from the Genoa side with Senad Lulic hitting their sixth just after the hour before Immobile completed his double 20 minutes from time.
Lazio's 21st win of the campaign left the capital side in fourth but Roma, and a possible Champions League qualifying place, is now just five points ahead of them with three games left to play.
Roma, 10 points behind leaders Juventus and two behind Napoli, visit the San Siro later Sunday where they can expect to meet a fired-up AC Milan after the Rossoneri were given extra incentive in their bid to claim a Europa League place.
In sixth place at six points behind Atalanta, Milan were buoyed after the men from Bergamo were held 1-1 by Udinese earlier.
Vincenzo Montella's side were probably even happier by seeing city rivals Inter stretch their winless run to seven games after succumbing 1-0 at Genoa after a 70th minute strike by Goran Pandev.
It was Inter's third defeat on the trot, their fifth from their past seven games and left them in seventh place at nine points behind Atalanta and with virtually no chance of qualifying for Europe.
Keita Balde has been enjoying a rich vein of form, scoring twice last week in a win that ruined what was Roma icon Francesco Totti's final derby as a player.
And the Senegal international shows no signs of letting up amid reports he is desperate to fashion a move away from the club.
After just two minutes, Balde fired Lazio in front from a difficult angle.
Lazio 'keeper Thomas Strakosha pulled off a great double save on Milan Djuricic and Karol Linetty's strike from the rebound, but minutes later Lazio had doubled their lead.
Balde sprinted into the area and after being hauled down by Milan Skriniar, Immobile stepped up to fire the spot-kick past Christian Puggioni.
Samp looked to be on their way back when Bartosz Bereszynski rolled across for Linetty, unmarked at the back post, to tap the ball past Strakosha.
But Lazio hit three in succession, Wesley Hoedt heading home a corner from point-blank range before Felipe Anderson beat Puggio with a penalty after being fouled by Vasco Regini.
Lazio made it five on the stroke of half-time when Dutch defender Stefan De Vrij coolly placed his shot into the bottom corner from Lucas Biglia's cut-back.
To their credit, Lazio kept the pressure on, Lulic hitting the hosts' sixth on 65 minutes and Immobile finishing off a one-two with Balde to beat Puggioni from a tight angle.
It was the right decision as a defiant Samp reduced arrears with two goals from striker Fabio Quagliarella, one from the spot, in the closing 18 minutes.