Pete, I believe Decamber is from Sweden. English is not his first language.
Decamber, for the occasional silliness, if you want to stamp it out, the best thing is to focus on that person for a short while - make parts of the adventure relevant to them. If they play it silly still, then their character suffers repercussions from it. Try not to kill them, but be sure that any flip answer is taken seriously and backfires on them.
If that doesn't work, then you might want to talk with the group and find out how the group in general feels about the game at hand. Sometimes you need to call attention to something to get the group to think about it.