Related post on Meta SE: How do you delete a room on chat?

Related chat FAQ: Will these rooms exist forever?

Quote (emphasis mine):

Rooms will exist indefinitely, so long as there is at least one person actively talking in the room. A room is considered worth retaining if it has more than 15 messages by at least 2 users.

Rooms not worth retaining which are inactive for 7 days will be deleted. Rooms worth retaining which are inactive for 14 days will be frozen. Frozen rooms do not allow any new messages to be sent, and are not shown in the default room list to prevent cluttering the rooms interface.

TL;DR: Post nothing and it will be cleaned up automatically in a week.

(terdon already deleted this one manually though, as mention in his own answer)

Related post on Meta SE: How do you delete a room on chat?

Related chat FAQ: Will these rooms exist forever?

Quote (emphasis mine):

Rooms will exist indefinitely, so long as there is at least one person actively talking in the room. A room is considered worth retaining if it has more than 15 messages by at least 2 users.

Rooms not worth retaining which are inactive for 7 days will be deleted. Rooms worth retaining which are inactive for 14 days will be frozen. Frozen rooms do not allow any new messages to be sent, and are not shown in the default room list to prevent cluttering the rooms interface.

TL;DR: Post nothing and it will be cleaned up automatically in a week.

(terdon already deleted this one manually though, as mentioned in his own answer)