Like "Stop it 5" or "Stop it, man" it means "stop that crazy, ridiculous, fucked up thing you're saying" -- with some added flourish. And like the "5" in Stop it 5," the "B" may once have referred to the Bloods street gang, or just may just be short for "blood."
Stop it B is also a popular catchphrase used by comedian Felonious Munk.
"Stop it B... You ain't all that, and none o' those shorties are interested in your broke ass."
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I think I'm gonna vote for Michelle Bachmann. them crazy eyes is sexy.
The phrase teachers and elders have taught the younglings.
Fun fact: This phrase utterly disrupts the bullies entire body, shutting down the physical and mental aspect and causing them to leave you online.
*Bully Beating a Kid*
Kid: Stop It I don't like it!
Bully: Yeah ok, you won this time...