Excluding from Default but Still Leaving Active??

For my default posting method (no @ or # tags), I do status updates to all my networks except Google Talk, AIM, and LinkedIn. I've added those accounts to that I *can* post to them from time to time, but for some reason I can't do @gt or @ai (etc) to get the update sent thru! Is there a way to exclude certain networks from my "default" configuration but still allow them to receive specifically-tagged messages??