Why should it kill open WiFi? Just initially firewall 25 and use a "captive portal" (http://www.personaltelco.net/index.cgi/CaptivePortal) or something similar to let connections through according to "acceptable use" criteria of your choice.

My personal incliniation would be to allocate WiFi users N "smtp credits" per hour, or day, or something, where they spend one credit per message per recipient. Set it to something large enough it's not going to trouble normal users (100? 500?), but low enough that it's not going to be worth spammers time and, worse, energy (spammers don't strike me as the athletic type) traipsing down to a WiFi hotspot only to have to traipse off to the next one after 30 seconds, and then the next, because they've blown their entire "budget". If they max out an entire city of WiFi hotspots they've probably still only sent 0.001% of the emails they'd send from home.

Or, perhaps require a validatable email address (ie email a PIN, get them to type it in), and CC: all email they send to it, so if they spam, they spam themselves. If one of your CC'd mails bounces with mailbox full, you stop allowing them to send SMTP. So they can't just throw it at a disposable hotmail account, cuz that'll soon fill up.

That's just two random schemes off the top of my head without really thinking about it, they're probably not secure long-term but they'd keep people off your back for a while -- but I'm sure the combined weight of the WiFi movement will crack it shortly. If it hasn't already, on a webpage out there somewhere I haven't read yet.