1) Reporting spammer to the source network does not usually help. However, if you really want to report, you can note down the IP of the spammer [ your form or application should have the ability to record the IP, if not please check with the developer of that application ]
You can then visit www.arin.net and check the owner network by checking the whois record for that IP. If the owner is apnic, ripe or lacnic, you wil need to visit their site to find the owner of that IP. Here is some relevant information: http://www.arin.net/abuse.html
Once you have found the abuse email address of the IP owner, you can report them the incident with full details.
2) The real solution is to protect your site from such attacks. You can do the following steps:
a) Add Captcha on your form to validate that a human is submitting the form. More details at: http://en.wikipedia.org/wiki/Captcha
Also check with the developer to add this feature or find a better script that comes with this feature.
Most spamming activity is done by automatic bots, and this will protect against those attacks.
b) Block the IP of the offender from your control panel.
c) Give the ability to add comments to guestbook/blog to registered users only.
We recommend solution (a) as the most appropriate, however depending on your own situation and your own decision on the usability of the site, you can do (b) and (c) as well to protect your site.