Simple explanation, valid for Marketo forms and almost all forms out there: modern applications rely on JavaScript for field and form validation, effectively moving that workload to the browser, reducing the strain on the server.
Disabling JavaScript at browser level means all validation rules are no longer executed therefore the form can be submitted without any information. Users have that ability. It is beyond the reach of web applications.
The solution is simple: add the noscript tag, redirectling to an invalid URL, discouraging users (mostly spammers) trying again.
I would suggest adding noscript to Landing Page Options - Edit Page Meta Tags - Custom Head HTML
You can use a real URL, say, the homepage, or to really disperse spammers, send to an invalid location to force a HTTP 404, very annoying to robots:
<noscript>Please enable JavaScript!
<META HTTP-EQUIV="Refresh" CONTENT="0;URL=http://bye_spammer">
</noscript>
Reference
http://www.w3schools.com/tags/tag_noscript.asp