To tell somebody not to do something you can use the infinitive, unless you want to specifically address a group of people:
- Non chiudere la porta! (little brother)
- Non chiudete la porta! (the whole family)
Otherwise, this sentence could in fact work for: Close the door (little brother)!
Thanks Mukkapazza. I would like to add that it is not always the case that the imperative and indicative forms are identical in affirmative sentences. Verbs ending in -are follow another rule. http://italian.about.com/library/weekly/aa011900a.htm
I can't wait for the imperative lesson in Duolingo ;)