The legal way, and this is something that's been pointed out in every single Soylent thread, is to stop selling it as a sole source of nutrition
You can do that, but you then need to meet the regulatory hurdles.
Or you can market it for the way it's actually used by the vast majority of people - a meal replacement.
Advocating a diet (or any other opinion) is protected under Section 2 of the Canadian Charter of Rights and Freedoms. Actually selling a product with potentially misleading labels is not.
There are many diets suiting many different lifestyles in Canada, from the traditional Inuit meat & fat heavy diet to many 'fad' diets extolled in magazines. It seems strange to me why it's legal to advocate a diet in media but illegal to label a product as such.