If the game was just you messing around, then it might be OK to reuse it after deleting all the threads. People may be confused about the game's creation date and other such oddities, though. If there were other players in the game it might be considered rude to delete?
There is not way to delete a game, so you will have to retire it. You can unretire it later if you change you mind.
Many people like to keep a game around for testing things, no need to keep making new ones each time.
Similarly, reusing characters that have posted in other games is probably bad form. Their name, portrait, and sheet contents will change everywhere and that could confuse other players. (Again, if you only used them in your own, private games, this does not matter.)