Ok... i was curious... so i just spent the past 30 minutes trying this... Here is what you do!
First, you can single them out real easily by making a "Smart Group" and putting "IM username is not set and phone is set"
Then you will have to create a new regular group where you will put all these contacts because smart groups dont seem to show up in iTunes. Highlight all the users in the smart group and put them in this new group.
Then in iTunes you can choose to only sync the groups you want...
http://www.bioinformatics.org/~deepan/freeoniphone/outlookimg2.png
There is also a setting there to add all new users created on the iPhone to be a part of a specific group.
Another option: Delete them all out of your address book and don't store their name in the "Address Card" section of iChat. They only get added to your contacts whenever you type in a name for them there. (Do the smart group the opposite way (phone is not set and IM is set) to find them all)
Good luck?