If your top-right corner looks like this, with the blue colorful clouds, you're signed in to an A365 workspace.

I mention this because, for the life of me, I can't figure out how to stay signed out.
If you click that settings icon and go to Data Management > Servers, you can turn off any workspaces you're signed into. If I do this, I notice that my components panel now still has all the same workspace components, but there's a note at the bottom that says I'm working on cached data. Note that at the bottom of that "servers" section, there's a button to "clear cache." The only problem is, "clear cache" instructs you to restart altium for it to take effect, and every time I restart altium, I get signed back in to my workspace.
I tried disconnecting the network to my virtual machine, and indeed after signing out, clearing cache, disconnecting network, and restarting Altium, I have ONLY file-based libraries in my components panel.
So here are a couple possible solutions for you:
1) sign out of your workspace, clear cache as above, and figure out what I couldn't about staying signed out (make sure you never get a blue cloud again in the top-right of the window). Maybe you've already got that sorted, and clearing cache is all you need to do.
2) If you truly don't want/use/need Altium 365 and want all the cloud data management stuff to just disappear, you could go to
https://accounts.live.altium.com/, find whatever workspace(s) you activated, and delete them there. I have to assume that'll take care of the components for you, perhaps with a "clear cache" as well.
3) If you MAYBE want A365 features and you're just annoyed about all those demo components you didn't make, you could go to
https://YOURWORKSPACENAME.365.altium.com/SettingsManagement#dataCleanup and just blanket delete them all from there. Then ONLY components you make going forward would show up. And of course, you can also delete whatever from the "explorer" panel inside altium as well.