Need help with form mail spam filtering.

Hi everyone. I've been using some asp scripts on my website for quite some time, but I've never really written my own so I'm kind of new to all this. But for all I know the solution to my problem might be a snap to one of your folks, and perhaps this could be a fun little challenge.

I am essentially using this form mail script on my site, with a few minor alterations.
http://www.brainjar.com/asp/formmail/

Every day I get spam e-mails from bots sending through the contact page. This has been going on for several months, bringing in hundreds of spam e-mails a week to me, and I'm fed up with it. There are different e-mail addresses and different messages, but I can tell that they all come from the same source because their e-mail addresses sort of follow a random pattern.

For the sake of example, lets just say I get e-mail from the following addresses.

MIKE@BANANA.FR
BILL@BANANA.US
JOHN@BANANA.COM
MIKE@GRAPE.CA
BILL@GRAPE.DE
JOHN@GRAPE.NET

So while the e-mail address themself change, there are obviously things in common.

What I want to do is add something to my form mail script that looks at the inputed e-mail address and checks to see if there are certain words there, and if one of those words is present, it will not send the e-mail.

In this case, I could individually prevent MIKE@BANANA.FR and MIKE@GRAPE.CA and so on from sending, but since there are so many variations, wouldn't it be so much easier to just stop any e-mail that contains BANANA or GRAPE? So that's what I want to add to the form mail script I'm using.

I hope I've made myself easy to understand here. There may be other ways to combat this, but I'm 100% possitive that if I could block any e-mail addresses that contain one of a short list of words after the @ and before the dot, I could bring my spam down to a screeching halt. So I know exactly what needs to be done to stop the spam, I just don't know enough about asp to impliment it, so I must call on the experts for assistance. I will be thankfull for any help.