While I take the point, equally that's not really related to the bug report. There is not concept of limiting the ability to edit a prefix entirely; the prefix is roughly considered to be part of the title, so the permissions piggyback on that, plus an additional level to disable specific options.
There are potentially use cases where a user changing the prefix from one they can't use is actually totally reasonable and expected. Taking your example in a different light, a staff member applying "pending user reply" would be reasonable (with the user being able to set that), but it would also be reasonable for a user to inherently remove that label. So what you're after is really something that fits more of a suggestion as it doesn't apply in all cases.
In the example in your last post, it should be resolvable by requiring selection of a prefix (which I have just enabled for the resolved bug forums here).