Does naltrexone cancel out the real effectiveness of ativan (lorazepam)?

Probably not. Naltrexone interferes with the actions of opiates and opioids. Ativan is a benzodiazepine (similar to valium), which is inhibited by flumazenil (clonazepam). Naltrexone and benzos are sometimes used together in treating various sorts of drug addictions.
It should not. Naltrexone blocks opiod receptors, whereas Ativan (lorazepam) works on GABA receptors, that are different from the opiod ones.