Sort of using somebody else's sample, yes, your best bet is to dilute or contaminate it and hope they get tired of asking you to retest. About the only advise I can really give you is to not sweat a lot before the test like an above post mentioned. These particular compounds stay in your fat cells, and if you suddenly start burning fat before the test.. guess what ends up in your bloodstream? All those compounds that were stored in the fat. Something like exercise is the last thing you wanna do before a drug test.
Enzyme tests are cheap and common now, and I don't think I've ever heard of anything that can actually fool them reliably. When it comes to any test, really.. the technology involved is outrageous. Hospitals spend many millions of dollars on machines that do nothing but analyze compounds in various states of metabolism and they can tell not only what you've taken, but roughly how long ago you took it in many cases.