I have a weird issue. I deleted a page and permanently deleted in from the trash but I can still get to it. On the deleted page it shows the posts page ("Blog" page). I have cleared the permalinks, cleared my browser cache and I have looked at it in incognito mode but I can still see the page. Other than turning plugins on and off any other ideas would be appreciated. I am also using a custom theme.
You could try redirecting the old url to your homepage (or another appropriate page on your site).
If you don't know how to setup redirects yourself, you can use a plugin like https://redirection.me/
Also, check to see if the page is still indexed in Google by searching "site:yourdomain.com/yourpage"