How to uninstall software that is running when the mac logs in
Check the Activity Monitor first to see if the App is running.
( Applications > Utilities > Activity Monitor.app )

If you can/can not find it there, it may be a launchd-process that has some special name.

Open Finder and hit CMD+SHIFT+G. Check these Folders:


/Library/LaunchDaemons
/Library/LaunchAgents


Delete the file you are looking for (App-Name / Company)
but be careful - do not delete any random file.

Restart your Mac.
If it still popp up, it is hidden somewhere else.
I don't know what you installed, maybe it needed admin priviliges and is therefore located in the System/Library/LaunchDaemons folder.

Other Folders that may contain your app/script.


/Library/StartupItems
/System/Library/LaunchDaemons
/System/Library/LaunchAgents


You could also check the logs via the Console Application.