Then you will have premium listings in the directory. Businesses will always try to increase their revenue and advertising is and will be a tempting option for them.
Reality is more complex. Organizing in solidarity is hard, deviation is easy… the whole non-cooperative games theories etc. Markets aren't all zero-sum.
But a bunch of restaurants in a town could all just promote the idea of eating out and promote their directory, and if no premium access or other advertising happens, then it will be best for everyone.
The core point is to reject the simplistic argument that ads are all just information and somehow fine and productive. Maybe you just meant that, in practice, businesses need to advertise. But if we get rid of a whole form of advertising, it can just help businesses rather than harm them. Each additional means to advertise creates yet more burden in a zero-sum race-to-the-bottom fashion.
Google lists plain facts like the existence of a business without any advertising even. If all the restaurants in a town decided in solidarity to never advertise on Google, people could still look up the restaurant directory and learn about the options. All the restaurants would save money. But when some advertise at Google and got prominence, they capture more of the market, which forces others to do more advertising in order to not lose market share.
And a restaurant having their own website is informational, but that's not what we mean by online advertising. We mean when third parties pay to have their ad promoted to people who aren't directly connecting with their business.
If all third-party advertising went away, it would be such a shock to the internet economy that it's hard to describe. But ignoring that aspect, we would see net GAINS in new businesses being able to have people notice their products or in customers being able to find the products and resources they want.