You can’t cancel an order yourself without it affecting your completion rate. Only customer support can do this - however CS would first need to rule that you are ‘in the right’ and that your buyer was wrong. Although the chain of events might appear obvious to you, you are still reliant on CS agreeing.
A quick tip (although some others might disagree). If you haven’t done so already, offer your client another revision. They will probably ignore this or say no (so you won’t be investing any more time). The reason being is that you can then demonstrate to customer support that you offered your buyer a revision and they either ignored or refused your suggestion. This should work in your favour.
No, you cannot offer a partial refund. This option does not exist on Fiverr - and with very good reason. Many greedy buyers would say things like “the work is okay but it’s not as good as I was expecting - give me 50% off and I will leave you good feedback”.
One more thought. Never ‘over deliver’ on what is promised in your gig. Only give your buyer what they paid for. 1) your buyer knew what they had purchased. 2) why waste your time on extra workload? 3) in the event of a complaint - you then have extra options. Had you only delivered two concepts to start with (and not four) - you could have used the extra original time you invested to deal with the complaint. 4) you are devaluing your own product by ‘over delivering’. 5) what if this buyer turned out to be a good buyer and ordered again in future? - they would expect you to over deliver again, further wasting your time. 6) we all know that scam buyers exist, so by always ‘over delivering’ - sooner or later (as you have found out) you will hit a scam buyer - so why automatically give up extra free time and your skills?