I used to deal with spamhaus during my DSN days. They were always reasonable, even when they knew Ed was playing games. For whatever reason they gave me an easy enough time and didn't hold it against us when we finally got the hell out of there.
I know it's a strong marketing tool, but you offering a /29+ with a VM for pennies isn't worth it. Spammers can't buy a /29 for $5, nevermind a /28 that you did for $7.50/month or something like that. Really, if they sign up and get 2 - 3 days of pure pound, they'll never charge back on you.
When we first opened up BuyVM on LEB we got a spammer signup within 24 hours. He was terminated 48 hours after that and never charged back. He never disputed it, argued it, made threats. He hit and run and our SBL was gone pretty quick, too.
Francisco