There are 3 types of email that we generally call spam:

Type 1: Bonafide Messaging with a real Commercial or non-profit(ie
political) purpose.  This category also includes contraband (eg drugs)   
which may be illegal in some or all juridictions, so long as the
advertiser actually intends to deliver the illegal goods. This also 
includes solicited and unsolicited bulk commercial email, though it may be
useful to distinguish solicited as Type 1A and unsolicted as Type 1B.

Type 2: Bonafide fraudulent activity. Someone is really trying to get your
money, but has no intentions of honoring their obligations to the purchase
contract.  This includes bonafide attempts at identify theft.  This is  
already criminalized by mail fraud and wire fraud, and other laws
concerning fraud.

Type 3: Annoyance activity. This has no bonafide intention of getting
money or even personal information, even though at a casual glance it may
appear so.  Type 3 is broken into 2 subtypes:

Type 3A is a relatively harmless disgruntled person, who is not terribly
sophisticated in their abuse, or in hiding their tracks. This type can be
handled by warnings or account termination. Besides spam, this type is
also involved in small DOS attacks and other unsophisticated abuse.

Type 3B is frequently a career criminal using viruses and rooted machines
to conduct annoyance to the point of extortion, which is frequently just
another type of DOS attack, but targeted perhaps at an email address, or
perhaps at a domain. This type of attacker is frequently already a career
criminal, having broken into many, often hundreds of computers, illegally.
This type cannot be dealt with effectively by ISPs, because they are
reasonable adept at hiding their tracks by crossing organizational
boundaries. Usually, the ISP only detects the infected computer, but does
not identify or catch the cracker.