General:
* Avoid
* Stop
* Hinder
* Thwart
* Obviate
* Preclude
* Avert
* Forestall
* Check
* Counteract
More forceful:
* Banish
* Curtail
* Suppress
* Prohibit
* Inhibit
* Deter
More formal:
* Preempt
* Anticipate
* Neutralize
* Mitigate
More specific:
* Discourage (to make someone less likely to do something)
* Disrupt (to interrupt or break up)
* Head off (to stop something before it happens)
* Sidestep (to avoid something)
The best synonym for "prevent" depends on the specific context. Consider the tone, level of formality, and the specific action you want to convey.