I have disabled the plugin, but the Google Analytics tracking code is still added to the page.
This question has been asked many times and in ALL cases it was caused by a Full Page Caching plugin, such as WP Fastest Cache or WP Super Cache, or another caching mechanism, such as Varnish. Possibly the page containing the snippet was cached in its entirety, and that cached version is still loading. Try flushing the plugin's cache and empty your browser's cache. Then reload the page.