Hidden fields seem to work pretty well: https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...
I then switched the field to a hashed simple time-based value, to which they responded to by just fetching the page to get the value, and posting it back.
The posted content was often in two categories: links with stuffed keywords, or some common framework exploit (generally fetching a remote resource to test for exploitability).
While marginally entertaining in the beginning, it's just a waste of time unless you want to create some form of engagement within a blog..
Don’t they usually set a value on the hidden field? One approach is to check that no value is submitted for the field.
It is pretty effective in most cases at evading most bots and dumb attempts, but it's also trivial to defeat if you're being targeted.