Gianni Versace must be turning in his grave over the goings-on at his fabled South Beach manse. The landmark Casa Casuarina — which new owners had run as a private club, restaurant and party palace — has been abruptly shuttered in the midst of an alleged $1 billion Ponzi scheme probe against one of its investors, Scott Rothstein. Casa Casuarina owner Peter Loftin slammed shut the gates after Rothstein, who faces up to 100 years in prison if convicted on all counts, was arrested this week. The Ocean Drive property where Versace was shot dead in 1997 was already on shaky financial ground. In February, the feds slapped Loftin with a $66,000 tax lien on the mansion. When Loftin bought the place in 2000, he’d boasted of making it a luxe club where members would shell out a $50,000 fee. But, by 2005, it was rented out for private parties, and one floor was converted into hotel rooms. Loftin didn’t respond to our calls.