There's not anything you can do, and that's the point of the test.
THC byproducts remain in the hair for over 90 days. When they say the test is designed to detect THC, they are not assuming that people are being good citizens and will ignorantly walk in for that exam. They are assuming that people who use drugs, will think just like you are, and will try to find a way to avoid detection. That is why a urine test isn't used to test for THC anymore. Hair is the prefect sample, usually taken from the eye-brow, since people do not completely shave their eye-brows, it can literally tell a lot about your life over the past 2-5 years.
One of the things it will definitely tell is your exposure to THC.
The good news is, most people are only casual smokers. Your hair is not a perfect database. It doesn't disclose whether you smoked marijuana once, twice, or five times, and whether that was just hours apart, or days apart, or months apart. It also doesn't disclose if you smoked marijuana at all. Simply inhaling smoke from a bystander is enough to show up in the results.
The test tries to be fair. If the amount is so negligible that it is possible that the amount can be due to second-hand inhalation 30 days ago, then it'll show up in the results, but it'll be marked negative (or, passed). But, if the amount is so obvious that it is likely or probable that the amount is due to first-person inhalation 30 days ago, then it'll stand out in the results, and it'll be marked positive (or, failed).