Ah yes...sync will still prevent gmail from adding these folks to your phone.
A couple options here aside from disabling sync...delete the email address from your gmail.com contacts if/When're youre on the computer.
Or
If you don't have any IMPORTANT contacts that you only have an email address from, you can set your contacts list on your Droid to display only contacts with phone numbers. That's what I do, because I don't email from the Droid TOO often, and if I do, its usually a reply. To do this, go to your contacts, press menu, display options, then check show only contacts with phone numbers.
Hope one of those options works for you!