How can I as a developer stop paying for apple ads?

Any campaign with a budget is “spent”, once the campaign has started.

https://searchads.apple.com/basic/

Apple Search Ads Advanced lets you set a budget, time period, and daily cap for each campaign you create. Together, these settings determine how long your ads run and how much you spend. Ads will run until you exhaust your budget, reach your campaign end date, or pause your campaign.

You can add to your campaign budget and adjust your daily cap at any time to ensure your ads keep running. Note that you can’t decrease your budget once your campaign has started. 

Stop bidding on the Apple search ads, most likely.

… Apple Developer Documentation

… Apple Developer Documentation