How do you stop white's queenside advance in the Reversed Sicilian? My openings textbook gives this line (Closed Variation of the Reversed Sicilian):

[FEN ""]

1.c4 e5 2.Nc3 Nc6 3.g3 g6 4.Bg2 Bg7 5.Nf3 d6 6.d3 Nf6 7.Rb1


with the note that the plan for white here would be to push 8. b4 and crush black on the queenside.

I want to stop that with black, so the first move I looked at was 7. ... a5, but I can't see how this doesn't fail to 8. a3. Still, this move is proposed in this answer and in this comment my question was asked, but it sadly went unanswered.

How do I stop b4?
You don't.

There is no way at this point to stop white from advancing on the queenside. Playing a5 still makes sense, because short term this will give you the open a-file and longterm this will eliminate a potential weakness that white might take advantage of once he breaks through. 

Instead of stoping white on the queenside you have to develop play on the kingside or potentially even in the centre to compensate for white's attack. 

Here is a nice and recent example:

[Event "World Cup"]
[Site "Khanty-Mansiysk RUS"]
[Date "2011.08.29"]
[EventDate "2011.08.28"]
[Round "1.2"]
[Result "0-1"]
[White "Arman Pashikian"]
[Black "Radoslaw Wojtaszek"]
[ECO "A24"]
[WhiteElo "2616"]
[BlackElo "2683"]
[PlyCount "64"]
[FEN ""]

1. c4 Nf6 2. Nc3 e5 3. g3 g6 4. Bg2 Bg7 5. Nf3 Nc6 6. d3 O-O
7. O-O d6 8. Rb1 a5 9. a3 h6 10. b4 axb4 11. axb4 Be6 12. Nd2
Qd7 13. b5 Nd8 14. Nd5 Nxd5 15. cxd5 Bh3 16. Nc4 Bxg2 17. Kxg2
b6 18. f4 exf4 19. gxf4 Qg4+ 20. Kh1 Ra2 21. Bd2 Nb7 22. f5
Qd4 23. Qb3 Rfa8 24. e3 Qh4 25. Qd1 Nc5 26. Qe2 Ra1 27. Rxa1
Rxa1 28. Be1 Qh3 29. Nd2 Bc3 30. Nf3 Ra2 31. Nd2 Bxd2 32. fxg6
Bxe1 0-1