General Antonyms:
* Prohibit: To forbid or prevent something.
* Forbid: To order someone not to do something.
* Deny: To refuse to give or grant something.
* Bar: To prevent someone from entering or doing something.
* Restrict: To limit something.
* Disallow: To not permit something.
More Specific Antonyms:
* Refuse: (When talking about permission)
* Prevent: (When talking about stopping something from happening)
* Obstruct: (When talking about hindering something)
* Impede: (When talking about slowing something down)
* Suppress: (When talking about stopping something from being expressed)
The best antonym will depend on the specific context in which "allow" is used.
For example, if you're talking about a parent allowing their child to stay up late, you might use "forbid" as an antonym. But if you're talking about a government allowing a new law, you might use "prohibit" or "disallow".