Contact form plugin that supports simple word filtering?

I’m using WordPress for a small business with three related sites that use a contact form.

We’ve managed to kill off most of the automated spam, but we’re still getting some manually-posted SEO rubbish. The obvious answer would be to filter out any messages that contain obvious spam phrases.

Most of the decent-looking contact form plugins I’ve come across seem to use Akismet filtering, including WordPress’s own Jetpack form.

As we’re a commercial business, this would cost us around $15 (3 x $5) a month- not silly money by any means, but still overkill for something we could probably achieve in five minutes via a simple word filter.

Are there any otherwise good (and secure) contact forms that include word filtering (i.e. that don’t force the commercial Akismet service as the only anti-spam option)?

– Raspberryade