If you describe something that someone says or writes as off topic, you mean that it is not relevant to the current discussion; used especially of discussions on the internet.
A troll is someone who posts inflammatory or off-topic messages in an online community.
off-topic in British English
(ˌɒfˈtɒpɪk)
adjective
informal
not related to the matter being discussed
We were inundated with messages about widely off-topic subjects.