Cannot remove advanced_Iframe

I installed advanced Iframe yesterday to try to resolve a specific issue. However, I think the issue was more with the other tools I was using (thrive themes) and so I’ve decided to rebuild using Oxygen.

I’ve therefore deactivated and deleted the plugin. I’ve cleared my site cache, my hosting cache and my browser cache.

However, trying to build templates in Oxygen, whenever I add their “Inner content” block, it immediately adds a block which shows an [advanced_iframe] shortcode

Help! How do I find and remove whatever remnant is causing this?

The page I need help with: [log in to see the link]