How do I stop background shortcut runner from running on iOS 18

In the Shortcuts app on your iPhone or iPad, tap Automation . Tap the automation you want to run automatically. Turn off Ask Before Running, then tap Don't Ask ...

Enable or disable a personal automation in Shortcuts on ...

I'm unclear whose device this is and who created these background shortcut runners. What leads you to believe that something is happening in the background? Are you speculating, or are you certain about it?

If you feel an unauthorized person/app is remotely using, controlling or monitoring your device, then that is possible only if you have done one or more of the following Don'ts...

1. Don't hand over an iPhone to kids or to a stranger without Enabling Guided Access
2. Don't share Apple IDs
3. Don't Jailbreak
4. Don't share sensitive information pertaining to your device
5. Don't give in to Phishing
6. Don't plug in your device in Airports and Public places through third-party cables and trust the device. Beware of Juice Jacking. (Especially in India)
7. Don't leave your iPhone unlocked and unattended in public places like offices, schools, malls, etc.

If one of the above is true then quickly change the Apple ID Password and Return iPhone settings to their defaults.