Multiple ways to quit a belligerent app in MacOS.

Force Quit (Cmd-Opt-Escape)

Activity Monitor: find the app, identify linked files. Delete the app and all linked files.

Browser extensions were notorious for hijacking browsers back in the day. They’d also throw up modal dialog with no clear way to remove them. But if any App Store developer did this Apple should consider them user hostile and cancel their account. There is no reason to create a dark pattern this evil.