He meant that Bobby would be out of the game, out of the business... the rant about Bobby was due to the fact that Bobby still had a soft spot for Uncle Junior even though Junior shot Tony. In Tony's mind, that's disloyalty, and the "Exile on Main Street" comment was Tony telling Janice that Bobby would be persona non grata (an unwelcome person) if he's going forward.
Remember Janice having a Rolling Stones Tattoo? So she exactly knew what Tony was referring to.
Exile on Main Street = Tony, referring to his planned rejection of Bobby for siding with Junior, says it's "Exile on Main Street" for him, the title of a 1972 album by The Rolling Stones.
In the HBO series The Sopranos, Tony uses the title phrase to let his sister Janice know her husband's disloyalty will never be forgiven: As for your husband Bobby, "Exile on Main Street." (Season 6, Episode 8 ["The Blue Comet"])
You could also say that Tony was making fun of Bobby's train hobby - next stop Main Street.
In the business world,
Main Street = Shorthand for "the investing public"--in the same way that "Wall Street" is used to refer to investment professionals and brokers.
In the Sopranos world, Wall Street = Mafia(-business)
-> Bobby out of the games of Wall Street
