Possibly, depending on when you want this "lock" to occur.
You can remove the field from the "edit" screen, but that removes any ability change the field in general - your users will only be able to change it during transitions.
Editing (and creating) comments is unrelated to screens (have a look at the permission scheme for the progect), and the ability to change status is controlled by conditions on your workflow.
JIRA doesn't have field level permissions per se. The only way I know of protecting a field from editing is to leave it off the edit screen. Then, you would create loopback transitions to each status (a transition that ends in the same status). On that transition, you could set your permission condition and expose the field on that transition screen.
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