The only option that comes to mind - find a user exit / BADI when saving the customer and there check if the field value was changed to remove this specific block (hopefully there are some variables that keep track of that) and then do authorization check and an error message if it fails.
If I'm understanding correctly that the users are allowed to change other blocks except for just this one, then disabling the field won't work actually, as well as transaction variants.
Although I have to say this is a very strange requirement, maybe it's not even a good design. I'd check with an SD person on what's the idea behind this. This looks suspiciously like a bad implementation of credit management...