If your Used capacity exceeds your Licensed capacity, no new writes to Swarm will be accepted until:
1) trapped space is trimmed during regular HP operations; 2) existing streams are deleted and the HP cycles through those deleted streams to trim the space; or 3) more licensed capacity is made available.
One possible scenario that can lead to Licensed capacity dropping to 0 is if you have a break in connectivity between subclusters. Say you have 10 TB of Licensed capacity and you have 2 subclusters, each with 10 TB of actual disk capacity. If your subclusters lose connection to each other, each subcluster could theoretically be filled up to more than 5 TB per subcluster. Once connectivity between the subclusters is restored and replication takes place between the subclusters, the total space used may exceed 10 TB. At that point, Swarm will issue an error about Licensed capacity and prevent new writes.