I agree with aetienne, I would continue to handle the boa..
He would consider himself being rewarded if you gave him what he wanted when he hissed at you (to be left alone). He would know he could scare you off that would encourage his behavior. I would not want to reward bad behaviour. I have one boa at home that is just as mean as spit. Every time it strikes the glass at me, I take it out. I don't think she is striking the glass quite as often anymore.
Two young ones I have ( around 3 feet) were extremely aggressive when I got them. I had to use gloves to handle them. One of them is improving. I actually was able to get through a photo shoot with only one attempted strike, instead of constant striking. I think that one will become workable before long...the other, I still wonder. With me, I've found that the larger a boa is that you are trying to tame, the less successful you will be. So, make sure you keep working with him while he is young. Keep in mind, some are just nasty and will always be nasty. But, if you've had this boa since it was a baby, and it has never been this way, chances are. if you are diligent, it will come back around. Trust me...nothing is scarier than a large boa hissing....I nearly wet my pants when my Suri started hissing...scared the stuffing out of me...I wasn't expecting it. She has only done it twice in the couple of years I've had her, but I forgive her a bad hair day once in awhile. 
Best of luck.
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Signature file edited; [phw 9/26/04]