I have encountered the same problem myself. I've switched to linking to an image of a new disposable email address. Zero spam since then. (Actually, zero non-test messages total since then, but that's fine too.) I suppose that will work only as long as not too many people do it, though.

However, there are legitimate companies hiring through those threads. I know someone who got their current job from one of those threads. If you can avoid the spam, I think the benefits outweigh the downsides.

Sadly I think it's any job board that can be easily scraped these days, have had very similar results when posting Reddit "for hire" listings