To add my .02
As CFC stated, we DO have some measures in place. The number of posts you guys see arriving pales in comparison to the number of accounts filtered out by our current software and add-ons. There are a handful of new accounts that get held for admin review (approve-kill) which we deal with daily. Then there are the damn few that make it all the way through to being activated and then spam us. These are traditionally real people that can pass most spam tests. The point here being we aren't exactly running with our pants down.
Second, we've got procedures in place to deal with such things. We typically request mods to only act in their assigned forums, but for a spammer they are encouraged to issue a warning wherever the spam occurs and the account is immediately suspended. This is to prevent further spam posts and generally catches such an account within the first few such posts. With our large community, members are very quick to Report Post (RP) such things to staff attention. With roughly 40ish staff members, we generally act on them quickly. What this doesn't do a good job of is cleaning up that post right at that moment (again, acting outside their forums).
The reason we allow mods to issue a warning that suspends the accounts, rather than ban, is in case there is a mistake in interpreting the post (generally there isn't, but a second set of eyes doesn't hurt). Additionally, we don't allow regular mods to ban anyone (lest we have member-mod issues that lead to abuse of power). Sr. Mods and Admin can ban, and for cases like these we typically use a SPAM tool that adds the offending accounts meta data (username, email address, IPs) to the great SPAM database in the sky. That db is shared by sites all over the world so with enough submissions, the site SPAM add-ons automatically ban registrations using those criteria (see the first paragraph).
Bottom line, we're not overrun. The occurrences are annoying, but few; and they are dealt with relatively quickly. This isn't' to say we're good where we are at, as improvement is always encouraged, and suggestions are welcome.
@CFC
Interesting Xenforo solution I found for bots.
CFC is no longer on staff. He's just more smart and charming than most of us on staff. Perhaps the 'smart' part has something to do with his moving to 'crew'. Don't count on him doing things, but it's always great to bounce ideas off him and get his input.
Your two suggestions, however, are intriguing. I'll check into them as they should be relatively easy to implement. Thank you very much for finding them.