There are instructions on the GitHub page https://github.com/arduino/arduino-create-agent for both completely uninstalling the Arduino Create Multi Platform Agent and for disabling its automatic startup.
You have asked how to do the latter. In brief, from the Finder:
- Click on the Go menu, and select 'Go to the Folder'
- Type ~/Library/LaunchAgents and press Enter
- Select the ArduinoCreateAgent.plist file
- Right click on the file name and select 'Move to Trash'
Alternatively, using Terminal.app, issue the command
launchctl unload ~/Library/LaunchAgents/ArduinoCreateAgent.plist
Using the second method of disabling automatic startup has two benefits: it retains the ability to easily re-enable it should you wish, while also allowing you the ability to start the process manually. To restore automatic startup, issue this command in Terminal:
launchctl load ~/Library/LaunchAgents/ArduinoCreateAgent.plist
To launch the process on a manual "at-will" basis: