in the
Update Manager under the
Edit menu option, you may be able to remove or disable brave in
Software Sources possibly in the
Additional Repositories tab. i've not used it so it might not show up there.
otherwise what looks like the official install page:
https://brave.com/linux/
shows a possible list for the source located at
/etc/apt/sources.list.d/brave-browser-release.list
you could try deleting that list if you are no longer using it.
editing to add: that's for mint in the initial section, but the source file should be the same for other debian and ubuntu derivatives. just realized this was the mint section
