HOW CAN I STOP PEOPLE FROM REGISTERING

How are they registering? Through a Marketo landing page? You could just redirect it to a "Sorry this event is full" page instead. 

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."

 

@Tania

Marketo has not capacity control feature.
https://community.marketo.com/MarketoIdeaDetail?id=08750000000KBJ1AAO