To prevent the Splice desktop app from automatically launching at startup, you'll need to disable that feature in the preferences of the app:

​

​

Next, if you're using Splice on a Mac: go to System Preferences > Users & Groups > Login Items. Then highlight Splice and click the minus ( - ) button to remove it as a login item. You may need to click the "lock" button and enter your administrator password before you can make changes to your system preferences.

​