No, there is not.
However, this link will show you how you can have Marketo tell you when milestones are hit so you can turn off the reg
http://hooshmarketing.com.au/blog/event-marketing/four-tips-to-building-a-limited-capacity-event-in-marketo
Essentially you run some lists and have it alert you when certain targets are hit.
Another option would be to run jquery to check the total registrants and change the form, but that sounds complex to me.