I've limited it to only covers created in 3D Ordi for regul…

I've limited it to only covers created in 3D Ordi for regular users, but if you're a subscriber to the platform, you can also add items to collections created in other apps (if you own the cover, of course).
The only way to prevent more items from being added to a collection in 3D Ordi is if no one owns the cover and it is burned. The collection will continue to exist, but the cover cannot be listed for sale or transferred, etc. Remember that the cover is also an ordinal, but with a different type of metadata.