Just leave it open. Flush it and clean it and all that jazz, but you don't have to shut it. Also, I'm not sure if it's relevant, but it never mentions if the cockroach was alive or dead...

My house is far less likely to become infested by stray turtles than is to become infested by stray cockroaches. The turtles might even prove to be valuable if I can find a buyer interested in a steady supply like that.