cancel it? cut the health in half?
Are you trying to change the meaning of existing commands, or trying to create your own commands? I’m still not 100% sure on what you are trying to do, but it sounds like you would be better off creating your own commands then intercepting things?
Possible reasons for wanting to intercept things would be to create your own selectors, or selector arguments, or change the behaviour of what players are selected, like not targeting players that are vanished or something.
But most other things I can think of could be solved by registering your own commands and using them in the command blocks / console. If you want to change the behaviour of vanilla commands, you could probably unregister them somehow, or create commands with the same alias. Then provide whatever functionality you want, or delegate to the original command callable.