Generally this is controlled by message no 06340. But I think this message has been set as warning by hard code (I've tried to make the message as error, but still it is showing as warning)
You can go with this below option if you do not want to allow user to change the value of these fields :
Make the field as display mode via transaction variant SHD0.
Then assign it to user id. Then user can't able to change these field's value.
