iCalendar (.ics) feed, how to force calendar client to stop the subscription

I have an online service where it lets people subscribe to iCalendar feed. I'm wondering if there is a way to force the subscriber (their Calendar client) to stop their subscription. I have some subscribers who subscribed to a feed that doesn't exist anymore. The annoying part is that these subscribers is fetching the data every 30 seconds!

I looked at iCalendar specification and it doesn't look like this is supported.

I tried few http response codes (404 and 410), but this doesn't stop the client.

The user agent I'm seeing in the log shows "iOS/17.4 (21E219) dataaccessd/1.0".

Any help is appreciated.

iCalendar (.ics) feed, how to force calendar client to stop the subscription
 
 
Q