How unfortunate. If bamboo were really as aggressive as the urban legends have convinced you, there wouldn't be room for any humans on earth. Would you have done the same thing if any other plant had put out roots through the drainage hole? Even my tomato starts do that! Yes, bamboo can get out of control if you plant them and ignore them for a number of years, but they can be contained easily with a little annual root pruning. Oh well, someone will be very happy to take those off your hands...