Nothing. It should work properly right now. That's why there are 2 questions for you:

1. When are you getting this error? On which page are you? Do you get it for all reviews? Etc.
2. Have you changed any browser security settings / installed any plugins?

Aside from answering these questions so we can find out what the bug is, you can't do anything right now.

Link:
Do you mean storing the code in the database? Because the code would need to be unique and usable only in one instance. Such an idea would work but would be a rather large undertaking.

It made me think of the following though: generate an md5 hash or something from the username + ip and check for that...