I'm not sure this is a bug or not because it based on what said on Minecraft.wiki.
So based on the wiki: "To remove a structure from active memory, players need to quit and reload their current game."

"Active memory" I think they meant is "save in memory" when using the structure block, but that didn't work for me.

Steps to Reproduce:

1. Get a structure block
2. Using structure block to save any structure in memory (save in memory)
3. Save the world, quit the game and reload again 

Expected Results:

The structure have been saved previous can't be load and complete been remove/delete

Observed Results:

Players can still load the structure and there is no way to remove the saved structure other than overlap the existing structure