Delete and recreate *2 subdomain then "You are already hosting this domain"

I created a subdomain: app.mydomain.com

I deleted it.

I recreated it.

I noticed a bunch of virtual servers appear, not just one or two, as "app.mydomain.com" so I deleted them all. Then the default server became "app.mydomain.com" and, of course, I couldn't delete it via virtualmin, but then I didn't really have to because when I restarted Apache to make sure I had a working baseline, it said that 000-default was linked to a nonexistent default under sites-available. OK, so I ln -s to ../sites-available/mydomain.com 000-default and was able to get the basic site back up. But when I tried to re-create app.mydomain.com yet again, it gave me "You are already hosting this domain". Global search of /etc/apache2 turned up one file referring to it: conf.d/security, so I reverted that file so it had no refs to app.mydomain.com and restarted Apache. It came up and I tried to recreated app.mydomain.com yet again and it still gives me "You are already hosting this domain". A global search from /etc turned up references to app.mydomain.com in components other than apache2-- references that virtualmin had put in those components -- but not in apache2.