If you delete a podcast from your ZenCast account, that show's RSS feed will immediately cease to exist.
If you cancel your ZenCast account entirely, the same thing will happen to all associated shows at the end of your current subscription period.
This means that over time, each distribution platform will eventually take note of the missing RSS feed and delete your shows and episodes from their library automatically.
In Spotify's case because they re-host or store their own copy of your podcast files, when the show is deleted from ZenCast, Spotify's copy can still exist. In order to have Spotify remove their copy, reach out to them via the contact options at https://podcasters.spotify.com/.
Occasionally, other platforms may also display content from your deleted show. The difference is that, unlike Spotify, most platforms will not store their own version of your files, so that content will no longer be playable even though the listing appears publicly. Again, the best course of action in this is to reach out to the platform responsible directly and ask them to remove the listing.