When I reply to people, Google adds them to “other contacts” in Gmail.
Unfortunately, people in this “group” sync to e.g. MobileMe.
Is there any way to prevent this?
This question isn't completely clear to me. But my understanding now is that you want to for example if your account is added to iPhone or something keep the sync from happening. The only way I can think of is the settings on the device itself. For example in iOS turn off the Contacts button in the settings for your Gmail account in Mail, Contacts, and Calendars. I have yet to see Google offer a solution to what I think is the question.