In cases like this, you can contact support and they will remove the unfair reviews. If the user repeats the offense, simply contact support again. I hear support will block them from posting reviews if they keep it up.
You can contact support here: http://enva.to/Help
I as a buyer sometimes forget to leave ratings until many months down the line, after my support has expired.
I always leave a review but if we locked down the review system to only active support subscriptions I wouldn’t be able to give my constructive (albeit quite delayed) feedback, which other buyers will benefit from. The author would also miss out on my 5-star rating. 
As an author, I’ve had many events where buyers left positive feedback even longer than a year after their purchase.
Since there’s many other reasons to leave a rating than support, I don’t believe this would be a smart move. You can still contact support about abusive reviews. They will remove them.
This is a little more tricky of a scenario, and probably a major influential factor as to why reviews were made public. We can publicly respond to reviews like this with something constructive and positive, such as:
Hello & thanks for the feedback. I don’t see any emails or comments from you about this issue. Can you please contact me [here] so we can get this sorted swiftly? Thanks!
This will show other buyers that you do work to support the item and that the review is invalid.
I personally disagree with this. There’s no foretelling what kind of situation will occur, where the author may be willing to provide a refund. The ability should be there, just in case.
The only time a buyer is eligible for a refund is when they’re within the refund policy. Since we can already decide whether to reject or approve a refund, there’s not really a reason to restrict it to the expiry of support.
The refund policy is here: https://themeforest.net/page/customer_refund_policy