Botulinum toxin blocks neuromuscular transmission by the following mechanism:

Ref:KD Tripathi pharmacology 7th edition (page.no: 99) Botulinum toxins A and B are highly potent exotoxins produced by Clostridium botulinum that is responsible for Botulism. These neurotoxic proteins cause long-lasting loss of cholinergic transmission by interacting with axonal protein involved in exocytic release of Ach and thereby closing the calcium channel in the pre synaptic membrane