Many list admins try to ban advertising completely. An outright ban is difficult to enforce.
Some subscribers have ads attached to their signature by the email service they use.
On most of my lists I allow brief and infrequent ads directly relating to the topic. Although I don't publish specific limits, anyone who posts an ad more than once a month or longer than a screenful gets a polite private note telling them to back off. Ads that directly relate to the topic are OK with me if they are infrequent and of modest length. Posting an entire catalog complete with shipping rates is absurd. A brief description of a product or service with a pointer to a web page or an offer of more info by private email is acceptable to me.