Spam - domain spoofing

I get tons of spam emails that are spoofing my own domain which is hosted on DiscountASP. I have looked into some methods to try to curtail this and they involve updating SPF records which I am not overly familiar with.

Is anybody else seeing this sort of spam? Since my domain only has a small handful of valid emails, I thought maybe I could blacklist my own domain but whitelist the valid emails...I'm not sure which will take precedence, however. I will have to do some testing.

Is there anything DiscountASP can do on their end to fix this up? Anybody have any other suggestions?

Thanks,
Dave