I use a dummy field on one site - called something like "Last Name" - the contents of which are hidden and must not be changed. The field contents are clear they must not be changed - "Do not alter this field!" - so that it still works for a wanted user if CSS has been tampered with.

No spam yet. But it's quite a small site, probably this is over only about 6Million hits.

For all I know it's just because it's a hand-coded site. Trying this on a WP site is on my todo list.