Currently there are only 2 ways in which new subclient associations to a Storage Policy are prevented:
If the Query and Insert time on the associated DDB is under-performing, new subclient associations are automatically disabled.
In this scenario, existing subclients will still continue to run as expected, but nothing new can be associated.
The option to ‘Hide and Disable Storage Policy for Backups’ is selected in Storage Policy properties.
While this will prevent new subclients from being associated, this would also prevent existing subclients from being able to back up, making this not ideal for you.
Currently, there is no way to achieve your end goal while the associated DDB is performing well. If there is a valid use-case for this, we can ask engineering to add an enhancement for this in the future.