Milan, who won their first championship since 2004 last weekend, delighted thousands of flag-waving fans in the main square by using the sign to remind Inter one of their 18 titles was won in a courtroom in the 2006 match-fixing scandal.
Inter, who went on to win four more titles in a row, finished third that year but were declared Serie A winners after Juventus were demoted in the 'Calciopoli' affair and second-placed Milan had points deducted.
Saturday's other game saw Lazio's Brazilian midfielder Hernanes score twice in a 4-2 home win over Genoa to lift his side into the fourth Champions League qualifying spot.
Another weekend of matches remain in Italy but the San Siro game meant nothing with Cagliari wallowing in mid-table so a party atmosphere was assured from the off, despite Milan missing out on a double by losing to Palermo in Tuesday's Cup semi.
Brazil's Robinho scored twice in the first half to add a samba rhythm to the festivities while Gennaro Gattuso, in bother for a foul-mouthed rant against Inter coach and ex-Milan boss Leonardo last weekend, popped up with a rare goal in between.
"I above all wanted the lads to finish off in style at home, in front of the fans and the club president who came to see his team, and they've done it in the best of ways," Allegri told reporters.
Club president Claudio Lotito was booed by his own fans during the Genoa game amid speculation the Romans may be forced to play European games in Florence next season due to unpaid bills at Rome's Stadio Olimpico.
