Fields are blocked or unblocked in field management section in admin. You can't flip that anywhere else, it's either on or off. The default handling of a blocked field is such that it will accept any first-time value, but then block any updates if they're from a blocked source (again, that is configured in field management).
So you want to request of your local admin that the field in question be blocked from updates, and you have a variety of sources to choose any/all of to block (you can block for flow-actions, or for list imports, or both, etc).
Search for blocked fields here in the community, that should help inform more.