If I were you I would probably use Segmentations to keep your non-marketing business targets separate from the rest. In addition, if the emails you send to them will truly always be operational - than another safeguard would be to set Marketing Suspended to True for all of them and populate a Marketing Suspended Reason with "Non-Prospect Business Contacts" or similar. Use a label that makes sense to you.
As for how to keep them from syncing, you'll need to add a filter to your smart campaign(s) that sync people that excludes them. If you create the Segmentation you can exclude the segment.
Hi Sam! A similar option would be to use a field to mark these records. From there, consistently using global marketable lists (that exclude these people), will help ensure these records do not receive other emails. Segmentations can sometimes cause system speed to go down, which is why I wanted to put this one out there as well!