When Earning Mode is Campaigns, open Admin > Settings > Settings > Earnings, set Allow Members Creating Campaigns to No, and save.
This prevents new member campaigns; it does not pause campaigns that already exist, and administrators can still create campaigns. To stop an existing campaign from serving, open Admin > Campaigns > List, use its action menu, and select Pause.
Changing the site to Simple earning mode is a monetization-model change, not a substitute for pausing selected campaigns. Record the current setup and test before changing modes on a production site.