#1 prevent v. [forestall, foreclose, preclude, forbid] keep from happening or arising; make impossible; "My sense of tact forbids an honest answer"; "Your role in the projects precludes your involvement in the competitive project". wn
#2 prevent v. [keep] stop (someone or something) from doing something or being in a certain state; "We must prevent the cancer from spreading"; "His snoring kept me from falling asleep"; "Keep the child from eating the marbles". wn
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