There are a few features built into the software to allow you to stop spamming through the software. Much of this spamming is done by humans manually clicking through the site. There is little that can be done to completely stop an actual human from using the site.

You can use the following feature to limit the number of communications any user can send through the contact seller feature:

E-MAIL SETUP > NOTIFICATION E-MAIL SETTINGS > CONTACT SELLER

This information is put at the bottom of every email sent to a seller through the contact seller feature. This is valuable for both you and your sellers to verify the communication is legit. Educate your sellers to this valuable feature. They can double check that the person contacting them is actually from the city, state and/or country they are communicating to the seller from. Many spammers/scammers pose to be local to the seller when in fact they are sending communications from Africa, Southeast Asia, Eastern Europe or anywhere you know someone wouldn't be contacting that seller about their item for sale.

You can also tell your clients to send you the ip addresses of these spammers/scammers and you can ban them from the site using the following admin tool:

SITE SETUP > IP BANNING

The best defense here is an educated seller. A seller will feel much better about your site if you make an effort to educate them about the tools you provide them as well as be proactive about finding spammers yourself.

While this doesn't stop humans from spamming through the site it does stop bots from doing so through your site. You can control where that appears in the site within the following admin tool:

ADDONS > SECURITY IMAGE > SETTINGS

You can add that to the contact seller form

Using the following admin tool you can get a copy of every email sent between sellers/buyers through the system. This at least gets your the first communication but cannot get a copy of an email sent from user to another outside the system. This copies you on all "contact seller" email and communique sent through the messaging system:

E-MAIL SETUP > GENERAL E-MAIL SETTINGS > BCC ADMIN ON USER COMMUNICATIONS

You may create a specific email account for this purpose so that you do not fill up your main admin email account.

This feature can be found in your admin tool here:

SITE SETUP > IP BANNING

This bans users using the IP's or blocks of IP's you insert from using your site. They can only see the front page of the software. You can find the IP for a user from the details of the contact seller email sent to the seller or within their user details within the USERS / USER GROUPS admin tool. The IP the user logged in from the last time is listed in their user details.

This feature allows you to limit access to features like listing placement and many others based on whether the user is verified or not. Basically until they are a verified user anything they do will require admin approval. You can find user verification settings within the following admin tool:

SITE SETUP > USER ACCOUNT SETTINGS 

And here within the support wiki.