The sphaleron interactions in the Standard Model (SM) is $(B-L)$ conserving and $(B+L)$ violating. Each sphaleron transition causes the change in the baryon number $\Delta B$ and the lepton number $\Delta L$ be such that $\Delta(B-L)=0$ but $\Delta(B+L)\neq 0$. These facts can be easily verified from the expressions of baryon and lepton current anomalies.
From this, however, it is not clear to me why is it said that sphaleron interactions will erase any initial baryon asymmetry? Such a statement can be found in the introduction of this paper "Baryogenesis without grand unification by Yanagida and Fukugita". I would like to understand this statement mathematically.