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All answers having "Prevent" as clue:

3 Character Answers:

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BAN

BAR

RHO



4 Character Answers:

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FOIL

KEEP

POST

STOP



5 Character Answers:

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AFTER

AVERT

DEBAR

DETER

ESTOP

STAVE



6 Character Answers:

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HINDER

KIBOSH

STYMIE

THWART



7 Character Answers:

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OBVIATE

RULEOUT



8 Character Answers:

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BLOCKADE

PRECLUDE

RESTRAIN

STAVEOFF



9 Character Answers:

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FORESTALL