A denial of service is just that - a denial of service. Whether it's done by pure gigabits of data flooding, more clever SYN attacks, or by crashing the opponent - it's denial of service. Asking a lot of people to call a phone number is in effect a distributed denial of service.
For the rest of your post,
1) how do you tell from just an email address that "this is a scumbag"?
2) (apart from instigating a DDOS probably being illegal) how do we have any guarantee that the phone number mentioned is a scammer?
3) for the email (dis)service, mailbombing is going to put load on mail carriers along the way, suck up bandwidth, and probably get sending IPs blacklisted.
Where do the machines form the mailbombing (dis)service come from? If it's rented boxes, there's likely some service provider that's going to be pissed off - if not that, the next unlucky customer that gets the same IPs and can't send (legitimate) mails because the IPs are blacklisted.
If it's from a zombie botnet, well... *rolleyes*