This is not possible today. The generic SCIM connector will only hard delete a user in the connected SCIM directory if the user is hard deleted in Azure AD while remaining in scope for provisioning.
For reference, the following actions all cause the IsSoftDeleted source attribute (which is calculated by our service) to return True:
1) User is disabled in AAD
2) User is soft-deleted in AAD
3) User is unassigned from the application AND provisioning is set to "Sync Assigned Users and Groups"
4) User is deemed out of scope based on scoping filters set on the Users mapping section
The default source mapping to the SCIM "active" attribute is Not([IsSoftDeleted]), or a Switch function that inverts True to return False and False to return True. Both the Switch function and the Not function achieve the same outcome.