I think, "unlawful" fits just fine; but you can also try different verbs like "rigging" (or, maybe, "tampering", although I am not so sure about this one) in order to convey the meaning of 不正操作. You do not have to necessarily stick to translating each of them separately ( 不正, then 操作), since there are several verbs and expression in English that can perfectly convey the meaning of the entire combination. This way you can keep the meaning and avoid tautology.

"This Libor-manipulation story grows crazier with each passing minute." http://www.rollingstone.com/politics/blogs/taibblog/libor-ba...

"The reputation of Barclays bank took an enormous blow during the Libor rate-fixing scandal, a YouGov poll reveals"http://www.guardian.co.uk/business/2012/sep/10/barclays-repu...

For "rigging": "Royal Bank of Scotland (RBS) faces fines for its part in a global interest rate rigging scandal and wants to reach a settlement as soon as possible.
US and UK authorities are investigating the part-nationalised bank over how it set Libor and other interest rates and it is expected to be one of the next to settle after British rival Barclays was fined $450 million in June." http://www.gulf-daily-news.com/NewsDetails.aspx?storyid=3410...