Like previous post said, just assign them to a different group in Google Contacts and choose not to display/sync that group.
Personally I just made an "Android" group and strictly monitor that group as it's the only group I sync from Google Contacts. Adding a new contact requires I go to the Google Contacts website and add them to that group manually if I want them on my sync list. After spending 3 hours cleaning up "My Contacts" when I first got my Epic, this step is nothing compared to having loads of erroneous contacts on my phone.
Also, selecting the option to only display contacts with phone numbers helps out with Facebook syncing if you happened to select sync all FB friends, rather than manually adding specific friends.