With the limited context provided, this is the best I can give you (more information about what the reviews are for, what they contain and who the users are):
I consider those two different use cases.
To amend a "bad" review (although I would encourage you to rethink the writing) with a change, explanation, new rating, … is excellent for transparency In the case, that the user needed his input to realize his "error" "
What happens if a user posts something by accident? If they make mistakes, if they do something wrong and publish without realizing it? What happens if they realize that they have misinterpreted something and want to eliminate the irrelevant and "bad" review? Do they have to write to you? I find that uncomfortable for the user (bad for them), and also much less likely to happen (bad for you).
Regardless of that, I think that users should be able to eliminate everything they create or publish. And in at least some cases, this is also legally required, did you prove it?