Something very simple that has worked for me for many years and across many smaller sites is a custom form field that requires some simple, static answer, like

Something very simple that has worked for me for many years and across many smaller sites is a custom form field that requires some simple, static answer, like "Please input 42 here:", plus even some Javascript that fills in that form element and hides it, so the typical user does not even see it (while it still works with Javascript disabled).