Do a search based upon whatever user name the guest posts are entered under ( I don't know what it is a I'm not in any manner encouraging anonymous posts on the my site)?
Personally, I don't see letting "guests" post directly to the site without account creation (which means they are no longer a guest), as you are simply asking for an influx of spam. Of course, if you are blocking massive blocks of IP's in an attempt to "stop" spam, I don't really see where letting guests posts is an improvement. I think the phrase oxymoron
enters the picture.