Remove Startup Arduino CreateAgent on High Sierra
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:


launchctl start ~/Library/LaunchAgents/ArduinoCreateAgent

[note that the .plist extension is omitted when using launchctl start]