Roz you say the following:

"Naughty of me I know but it is fun now and again to throw a virtual firework into the room sometimes and see what happens. Livens up a dull evening."

On a forum this is called "trolling":

"In Internet slang, a troll is someone who posts inflammatory, extraneous, or off-topic messages in an online community, such as an online discussion forum, chat room or blog, with the primary intent of provoking other users into a desired emotional response[1] or of otherwise disrupting normal on-topic discussion.[2] In addition to the offending poster, the noun “troll” can also refer to the provocative message itself, as in that was an excellent troll you posted. While the term troll and its associated action, trolling, are primarily associated with Internet discourse, media attention in recent years has made such labels highly subjective, with trolling being used to describe many intentionally provocative actions outside of an online context."