You have lots of options for segmenting your contacts and preventing various things from happening to them.
- You can add these contacts to a shared list, and manage the segment by managing the shared list. This would be an easy and natural way to do it - whenever these contacts are added to the system, either by upload or by the API, just add them to the shared list. Then when you create other campaigns, just exclude this shared list from your segments. To remove them from the shared list if they convert through another offer, you could add a "remove from shared contact list" form processing step to your other forms, so if anyone on the list fills out one of those forms, they are removed.
- You could do essentially the same thing by using a custom contact field. The advantage there is that it makes it more obvious who these contacts are (because they're marked on the actual contact record, so you'll be able to see if they are in this segment by just looking at their field values, for example in an exported CSV file). The disadvantage is that it uses one of your contact field allotment.
- You could manage it through email group subscription - that is, you could subscribe these contacts to a particular email group, and unsubscribe them from all of the others. That way it doesn't matter if they end up in your other campaigns, they won't get those emails anyway. If they convert through another offer anyway, you could toggle all of their group subscriptions (unsub them from their original group and subscribe them to all the others) in form processing steps. The advantage here is that you don't have to worry about excluding them from campaign segments, as they will be excluded from the emails automatically. However, I think this would be the most trouble to manage and I'm not sure how you would use it to control synch to Salesforce.