I believe it's also on Marketo's roadmap in the near future to add functionality that will automate what Justin is suggesting - i.e. as soon as capacity is reached (or after a specific date or something like this) the reg. form would disappear.
Not clear on the exact solution that is in the pipe, I'm just pretty sure I read about this in one of the recent monthly release news letters.
Anyway, I know this doesn't help your immediate situation if you've got a webinar currently open for registration, but you do pretty much have to monitor this manually for the time being. Usually I just end up removing the registration form from the landing page and replace it with a block of text that says "we're sorry, the event has reached capacity...etc. etc."