I would suggest creating a Akismet plugin as a replacement for the Captcha's

The advantages:

- users don't need to type a captcha
- so for users with less tech skills, it is much easier for them to fill out a contact form for instance
- spam is still kept out

see: www.akismet.com

implementing is quite easy, for instance see: https://github.com/achingbrain/php5-akismet