Georgina made a good start. We don �t use "warn against" with people, but we do use "warn about" with people, actions (gerund form) and objects.
To warn of is to give information about negative consequences of something: It needs some negative word (in this case "dangers") after it.
My father warned me of the dangers of marriage.
To warn against and warn about are the same, but it doesn �t need the negative word.
The surgeon general of the United States warns us against/about smoking. (You could also say "...warns us against the dangers of smoking")
To warn a person is followed by an infinitive, and usually "not" or an appropriate verb that doesn �t need "not".
I warned you not to drive with your eyes closed.
Her mother warned her to be wary of that man.
I don �t think we use "warn off".
Bruce