As per this news...
Apple Officially Reveals How Long Siri Keeps Your Data | The iPad Guide
...and specifically this quote:Does this mean that if users want to purge their Siri-related data from Apple servers, they just have to turn it off? And perhaps wait a few minutes, and then turn it back on to "start from scratch"?“If a user turns Siri off, both identifiers are deleted immediately along with any associated data.”
It sounds "too easy", but if it is so, it's a nice workaround for privacy oriented.