It's actually recommended to use multiple spam filters. Without going into too much detail (I don't have much time right now), my current setup is Spamblock Bee with honeypot, but no hidden captcha, then Spamblock, then Spamblock Bayes. I know a few people use a similar setup. Try searching the forums for recommended settings and tips, and of course feel free to ask for specific details.
If you mean Serendipity 1.6, you should update that to a current release as soon as possible.