Apple - How do I disable WebEx Assistant on startup for Mac?

Try System Prefs > User & Groups > Your name > Login Items.

Select it in the list & click the - button underneath

If it's not listed there, then check either /Library/LaunchDaemons/ or /Library/StartupItems/ to see whether it has an entry in there.
It's safe to remove items from there - though they will continue running until the next reboot.

If you want to be able to restore functionality in future, zip the file in situ then trash the original.

I tried removing this from the user's Login Items, but it re-appeared each time I started the app. I could not find it in any of system or user launch or startup .plist files.

Then I noticed that the "Preferences" menu, where this sort of thing would normally be configured was grayed out. So I went looking for Webex plist file itself.

Turns out if you look at ~/Library/Preferences/com.cisco.webexmeetingsapp.plist you'll find a key called PTLaunchAtLogin.

I used a text editor and changed it to "false". If the file is not readily editable directly and Xcode it's installed on your Mac, simply double click the .plist file and toggle the key to "false".

Problem solved.

<key>PTLaunchAtLogin</key>
<false/>