I have emailed (using either my phone or the GMAIL client online) that have been added to my contact list. I went to send a text message and when I went to search I saw like 45 or so email addresses that have been added to my contact list (only when I go to send a text, not in my "people" app) and what to get rid of them from displaying in my phone.
This may not be exactly what you are looking for but you may want to try Handcent. The list of names it pulls up is different and I think only goes by real names. I have an email that goes "srv..." and that comes up when I try to send a text with the messages app. When I use Handcent, nothing comes up once I get to the "sr" because I don't have any contacts with those first two letters.
The only other thing I can think of is to delete the email addresses from the Gmail contacts on the web. I think all contracts are synced, even the ones that aren't "my contacts."