Can I reject return request?

Was the item returned within the return window? and is it resaleable at all if you assemble it correctly?
If so then you could offer a partial refund to reflect the new value.
If the customer has rendered the item unsaleable and it was not in fact faulty at all then no, you are not obliged to refund, but do take photographic evidence to show Amazon exactly why you have declined to refund.
Communicate the reason with your evidence, to the buyer and to Amazon, and ask the customer what he would like to do about his property.