Well, Gmail is a bit special as it's (still) invite-only,
meaning you can't have, say, 10 different accounts, so ban evasion isn't easily possible.
Or is at most one-shot.
Spammers would be also very easy to block.
Gmail's Policies:
CODE
Prohibited Actions
In addition to (and/or as some examples of) the violations described in Section 3 of the Terms of Use, users may not:
<snip>
# Impersonate another person (via the use of an email address or otherwise) or otherwise misrepresent yourself or the source of any email.
Ban avoidance is misrepresenting oneself as person other than banned. Don't forget that IANAL.
CODE
<snip>
# Generate or facilitate unsolicited commercial email ("spam"). <snip>
I don't know whether abuse at gmail . com is valid, but...