Hello Lucas,
"We need to prevent all users to assign storage policies with primary FTT = 0 in streched cluster."
If you need to prevent users from using a type of Storage Policy then you would need to 1. Not have such policies available for selection and 2. Limit permissions to prevent users from just creating such policies and then using them.
"As these clusters are in the same vCenter, the policies are availabile on all datastores and clusters."
This doesn't mean they will all be usable by all cluster types - if you for instance create a Storage Policy with any of the 'Site Disaster Tolerance' types that say 'Stretched-Cluster' it will show non-stretched vsanDatastores as Incompatible (because they don't have exactly 2 Fault-Domains + a Witness).
Bob