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Once the field is updated with whatever value you would like, you can then block any field updates to that field. Navigate to Admin > Field Management > drag in the field in question > Field Actions > Block Field Updates > check all the checkboxes.
This is a field that will be on all contact records. For each individual contact I'd like it to lock as soon as it populates. If I just block field updates, the field will never populate, correct?
Well it will never update after the field is blocked, so in your case you won't be able to block the field until all updates to the field are done.
What you can do in the meantime is drag the field into the field management area of admin and you can at least see where the field is being used in Marketo ("Field Used By"), to ensure it doesn't get updated from within Marketo.
To the best of my knowledge, blocking field updates through admin will not let you update the field once it has been populated, you should still be able to update the blocked field if it is blank. So this option should serve the purpose