How to remove how to remove Subject Completion block? across the whole site instead of going by removing it from each subject?
I am not sure what the Subject Completion block is.
If you mean the Course completion status block, if you go to Site administration > Plugins > blocks > Manage blocks, you can Hide the blocks. This will remove them site wide. If you decide to show them again, those courses with it on will then have them turned back on. You would not be able to do this process however, and then add it to a particular course - it will be disabled on your site.
You could uninstall the plugin and reinstall it - that would remove all instances of it, but then allow it to be used again.