A 28-year-old man from Delhi sustains a puncture wound to his foot from a rusty nail while working in a construction site. He was last vaccinated against tetanus 12 years ago. Three weeks later, he presents with jaw stiffness, difficulty opening his mouth, and neck rigidity. On examination, he has a characteristic facial expression with raised eyebrows and clenched teeth ('risus sardonicus'). His reflexes are hyperactive, and he has spontaneous muscle spasms. What is the mechanism of action of the toxin responsible for this clinical presentation?