You can blacklist any contact by right-clicking on the contact's name within the Evercontact pane:

You can also blacklist any contact or domain (and review your blacklists) from within the Evercontact menu (see below for how to get there). 

How do I get there?   

File > Evercontact > Blacklists 

OUTLOOK 2007 

Menu > Tools > Options > Evercontact > Blacklists