General synonyms:
* Avert: implies actively stopping something from happening, often with urgency.
* Avoid: implies taking steps to steer clear of something.
* Block: implies physically stopping something from passing.
* Check: implies stopping something from progressing.
* Deter: implies discouraging something from happening.
* Hinder: implies slowing down or obstructing something.
* Obviate: implies making something unnecessary.
* Preclude: implies making something impossible.
More specific synonyms:
* Thwart: implies frustrating or defeating a plan.
* Circumvent: implies finding a way around something.
* Counteract: implies reversing the effects of something.
* Suppress: implies forcibly stopping something.
The best synonym will depend on the specific context and the intended meaning. For example, "preventing an accident" might be best expressed as "averting an accident," while "preventing someone from entering a building" might be best expressed as "blocking someone from entering a building."