Simple and easy answer: all modern web applications enforce validatiions at browser level to offload that task from servers.
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:
<META HTTP-EQUIV="Refresh" CONTENT="0;URL=http://bye_spammer">