I see people in my referal logs finding my site by typing "registration" at google and then going to the forums to register for the purpose of posting spam.

there is little reason for most webmasters to have registration pages indexed on google. I'd like to request that this page be hidden from search engine spiders

I dont think this can be done with robots.txt, which doesn't support parameters to cgi applications. anyway it should be done at the application level to ensure compliance

You can do this via robots.txt, yes.

You can do this via robots.txt, yes.

please shpw me an example. I don't think it is possible. they both use index.php.

User-agent: *
Disallow: /index.php?act=reg

User-agent: *
Disallow: /index.php?act=reg

I have read discussions on other sites regarding this and no one says that this will work. they specifically say that this does not work. therefor, it cannot be done this way. it needs to be done at the application level.

Can you give some sources on this? I've always used this and I've never seen my registration page show up in Google (not that I've spent a great deal of time searching, but searching for specifically my site never shows me the registration page).

http://www.robotstxt.org/wc/norobots.html#format

According to the specifications, it IS valid.

Try it in Google webmaster tools.