Fissle, I know the answer to this one myself.

If you shave your head, they'll ask to take a sample from your underarms, pubes or legs, if you have long-enough leg hair for a sample. If you shave everything so you can't provide a sample, the best case scenario is that they'd postpone you for testing until you grow more hair, assuming your potential job is that accommodating. But what really normally happens is that they'll regard your inability to provide a hair sample, especially through shaving, as a sign that you're a user trying to hide something and need to be taken out of contention for employment.

Read the link below. Cannabis is detectable in hair for up to a year. Fortunately, hair tests only examine the most recent inch and a half of hair for drug testing, which translates to only about 90 days of cannabis-detectability. Since you've smoked heavily and have done so as recently as within this last two or three weeks, you're going to be outta luck.
Erowid Cannabis Vault : Drug Tests

There's no way to flush your system so cannabis won't be detectable in your hair. Hair tests are the most impossible ones to pass.