The whole point of this being a web app is to make it as easy as possible for the user to download/modify/transfer their user data. LASIM is a traditional app the user has to download and install, similar to a script this web app was developed to replace due to being too difficult to use for some users.
The import functionality targeted by this API is additive and my app features a built-in editor to add, modify or remove information as the user sees fit. To achieve your stated goal, you’d have to remove anything except the blocked_users entries before importing, which my app supports, I added a wiki entry explaining the workflow in more Detail.
I may add options to modify the exported data in some ways via a simple checkbox in the future, but I wouldn’t count on it. I’m always open for pull requests!