You cannot disable SNS. If you don't want SNS, you have to model your data accordingly. The JCR spec explains (via the CND format or the API) how to create data types which don't allow SNS.
I wanted to do the same thing and found this node type definition. You can use it in your CND as your base node type instead of using nt:unstructured.
* A node type that does not allow same named siblings. Otherwise it is identical to nt:unstructured.
- * (undefined) multiple
- * (undefined)
+ * (nt:base) = xyz:baseNodeType version