The gmail/yahoo/hotmail ban is a PITA and no doubt keeps a lot of people out. It is a necessary evil though. If we did not have it you'd be wading thru hundredsViagra ads/ my sisters cousin makes $168/hr working on her computer at home/ vote for obama posts just to find out what kind of oil to use. It's been tried and did not work out well. Damned spammers! 
Maybe could allow those emails but add a short V4/Honda/motorcycle-specific questionnaire to register and maybe a Craigslist-style "This is a bot" button (5-15 members must click before a 72-hour ban, multiple occurrences means perma-ban)?
1. ATGATT means...
a. You have a venereal disease.
b. 5-25, courtesy of your local Department of Corrections.
c. Always wearing your personal protective equipment.
d. You attended Gattica High School.
2. If you owned a 1984 Honda Sabre and a 2004 Honda Sabre, which would have more cylinders?
a. The 1984, duh!
b. The 2004, cuz it's newer.
c. They are both the same.
d. What's a cylinder?
3. You come around a corner scraping pegs on your Magna and accidentally spray a large contingent of Hells Angels with gravel. You...
a. Wipe out because you weren't watching the road.
b. Smoothly roll on more throttle and leave them in the dust.
c. Are run down (hey man, they ride Harleys) and turned into a doo-rag rack in the clubhouse.
d. Were wrong... it was a meet-up group: the Vespa Vandals!
4. You walk out of the bar/diner/coffee shop and find your Interceptor won't start. What do you check first?
a. You make sure you aren't trying to start it with the keys to your new Audi.
b. You try to figure out who installed the sidecar while you were inside and realize you don't actually ride a Ural after all.
c. You check the kill switch, gear position, and the main fuse.
d. You pull out your Iphone and call for a tow.
5. You ride a first gen Honda Magna. You bike has a...
a. Chain drive.
b. Belt drive.
c. Shaft drive.
d. Sex drive.
Anyway, you get the idea.... easy to Google or intuit (depending on which way you like to go) but harder to program a bot for?