It is possible to adequately define and ban "negative" political ads. And here's how: The candidates should not be able to mention other candidates, or other political parties. They should only be able to mention their accomplishments. This will effectively end all "negative" political ads, because all negative political ads mention the other candidate or other political party. By limiting the ads to where only the candidates talk about themselves, this would do away with negative ads.
Almost every ad is, in a sense, negative. It is attempting to manipulate any individual who views it to think, feel, and act a certain way. Whether that was is designed to make one donate to a charitable cause or to buy alcohol is irrelevant. All ads are about manipulation, and so all are negative. Banning "negative" ads would be to ban all ads.