Derive the launchd config plist based on a launch label When I list the currently running services, I see a service I'd like to remove: 

% launchctl list | grep -i spotify
 -  13  com.spotify.webhelper

Launchctl allows me to stop it via launchctl stop com.spotify.webhelper but it does not allow me to completely remove it (uninstall it) unless I specify the .plist file.

How can I derive the plist based on the label?
locate com.spotify.webhelper should do the trick as long as the plist is properly named (and the update process for the locate db has at least run once since you installed Spotify). Otherwise have a look at /Library/LaunchAgents, /Library/LaunchDaemons and their equivalents in your own Library folder.