Fighting Comment Spam

I updated an install of WP to 1.5.2 last night. The comment spammers are already on to it! Presumably that means they have been trying it all along.

I find this:
https://codex.www.ads-software.com/Working_with_WordPress#Fighting_Comment_Spam

That describes a number of options for fighting comment spam. Previoiusly we had employed a captcha … but that required that code be inserted into the comment php files (wp-comments.php – add the captcha/run the script; and wp-comments-post.php – I ran a clean out of the captcha image on the spot so that it could not be reused once you posted … vs the authors suggested cron job to clean out the old images).

Is there a true plug-in solution that is good and doesn’t require that I make physical code modifications to the .php files? The problem is that, as near as I can see, those files are now part of the theme set up and as such, I would need to hack them every time I add a new theme?

Nothing here really addresses the issue I am raising, just many options … that I need to read through one at a time in an effort to find a solution.
https://codex.www.ads-software.com/Plugins/Spam_Tools

I guess a true plug-in solution would seem to me not to require the addition of code to individual files, but rather would take care of all that for me it seems to me.

Thanks for any help.