There only way Ratings Abuse can be reported is if the person who left the rating has been regularly abusing the ratings system. A single questionable rating is not something that can be reported. Individual ratings are not moderated. The generally accepted minimum standard of proof for ratings abuse reports is that you have to find 5 examples of obviously incorrect ratings left by that person. If you can do so then you may file a report in the Cheating & Abuse
area. The burden of proof is on the accuser in cases of ratings abuse, so you would need to find the examples and explain why they are obvious abuse in your report, or it will be closed.
Note, even if the person who left the rating is found guilty of ratings abuse the ratings they left will not be removed. The player will simply move up to the next level on the ratings abuse punishment scale. (Which ends with them no longer able to leave ratings.)
Accordingly to your description the rating left for you does not appear to be obvious abuse. If you disagree with it you can nicely ask the player who left it to withdraw it and he might do so. Otherwise just leave an appropriate response on the rating and move.