Simple and easy answer: all modern web applications enforce validatiions at browser level to offload that task from servers.
Validations to enforce required fields are executed by JavaScript. Spambots set JavaScript to disabled to bypass validations and increase their performance.
Simple solution: add <noscript> tag to landing pages containing forms.
Landing Page Actions - Edit Page Meta Tags - Edit 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