As Osvaldo said, TGFb1 and myostatin are well known myogenic inhibitors.
Rapamycin will also inhibit IGF-1 induced myotube hypertrophy and in some conditions the differentiation: see the paper "Skeletal myocyte hypertrophy requires mTOR kinase activity and S6K1" Article in Experimental Cell Research 309(1):211-9 · October 2005