Pretty sure you already fixed your problem, but what I did for this problem on my S7 Edge was I went into Touchwiz & Touchwiz Easy apps (these are the Samsung pre-installed stock launchers) and forced stopped/disabled them both. When you are doing that make sure you turn the Notifications OFF on both of those, as well!
I then went into my favorite downloaded launcher app (CM Launcher) and set it as default (if it won't let you do that, no worries) because either way, well at least for me this was true, the next time you hit the home key it will ask you to once again to select your default launcher... Select and Pick ALWAYS...
Now it should NOT keep asking you from that point forward (yet some things you open for the first time after that, such as email) may ask you one more time... But hopefully it should solve the problem for future use!! Hope this helps.