50 Cent is going scorched Earth on Floyd Mayweather -- repeatedly blasting the boxer for undermining the Gucci blackface boycott ... and it's getting personal.
Of course, Mayweather publicly flouted the boycott and dropped thousands of dollars at Gucci on Monday night ... despite calls from stars like T.I. and Spike Lee to stop supporting the brand.
They're upset over a Gucci sweater that depicts blackface -- with T.I. calling the clothing item straight "racist."
Mayweather told TMZ Sports he refuses to let other people tell him how to live -- and blew off the boycott as irrelevant to him.