Sorry, but you have paid for something and you have got something. If you are not happy with the product you have received, you can take the time to say so in your review. By asking your seller to cancel, you are treating Fiverr like a magical place where you get to make orders and only choose to pay for what you like best. After all, what happens if you don’t like the quality of the work your next seller produces?
Fiverr’s terms clearly state that you can not reject work because you personally do not like it any more than you can take back your McDonalds after deciding that you want to eat at Burger King.
More importantly, all asking buyers to cancel orders does is make them more likely to abandon working on Fiverr or increase their prices. After all, would you routinely work for a day only to have someone come back and say “hey sorry, we’ve decided that we’re not going to pay you.”
What’s fair and appropriate? Realizing that someone has spent time on your work and that they like you can’t survive off fairy dust.