When Earning Mode is Campaigns, open Admin > Settings > Settings > Earnings. Set Allow Members Creating Campaigns to No, and save.
This stops members from creating new campaigns. It does not pause campaigns that already exist, and administrators can still create campaigns. To stop an existing campaign, open Admin > Campaigns > List. Open 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.