1. Log users activity
You can't know what's happening on your server if you're not logging anything. Sometimes, you may not even know for months that something wrong is happening if you're never looking at logs.
When somebody does something wrong, you may want to find the concerned IP address or addresses. Then, use WHOIS to gather more information about the person.
2. Protect your forms against bots
CAPTCHA is helpful especially on such forms where anyone can post anything on your website. You are lucky that you're spammed with meaningless messages. More often, this spam contains links to porn websites and other sweet things. Do you really want to be associated in Google and other browser engines with those websites?
3. Protect your forms against morons
CAPTCHA will not prevent an human to spend a whole day posting manually some malicious comments. To prevent such behavior, you may want to limit the number of authorized postings by hour by IP. This will not prevent the person to post those comments while switching proxy servers, but will require more effort and time.
4. Review and accept
Before the content is actually posted on your website, you may also want to review it manually to be able to prevent any malicious comment to actually appear on your website. Again, this is a good idea if you actually care about your reputation in Google search.
If nothing works (for example a human is posting from different proxy servers), you may want to search for patterns which makes the difference between the spammer and other people. In your case, it would probably be the form of the messages and the number of special characters and digits compared to the number of small and capital letters.