An easy-to-implement but not fool-proof (especially on "specific" attacks) way of solving anti-spam is tracking the time between form-submit and page-load.
Bots request a page, parse the page and submit the form. This is fast.
Humans type in a URL, load the page, wait before the page is fully loaded, scroll down, read content, decide wether to comment/fill in the form, require time to fill in the form, and submit.
The difference in time can be subtle; and how to track this time without cookies requires some way of server-side database. This may be an impact in performance.
Also you need to tweak the threshold-time.
Last edited by davefranco; 12-03-2015 at 12:19 PM..