Negate INSERT

[Jorge Ferreira]

Hello everyone,
It is said in White Pappers that we should use triggers because:
...
You can use them to check for, and prevent, bad data entering the database.
You can change or negate the INSERT, UPDATE, and DELETE statements.
...
And that's ok, but how can i negate (deny) a INSERT, UPDATE or DELETE statment through a trigger?
Does anyone has an example of code to do it?

Thank you,
Jorge