Hello
Have you trued using "ALTER SYSTEM CANCEL SESSION" for these?
In relation to the "working on snapshot_ID" - the reason for this is the E-mail processing, may be blocked by MailSenderCallback semaphore waits, see SAP Note 1999998 for more details.
You can find more on this and other threads at:
2114710 - FAQ: SAP HANA Threads and Thread Samples
This should answer you question I hope.