Don't see a way to cancel and Siri doesn't delete Reminders on the Watch (at least not for me). The only way I see is to open the Reminders app and check it (or use your iPhone). Maybe someone knows a way.
"Hey Siri, set a reminder to do laundry at four o'clock."
Siri sets a reminder.
"Hey Siri, cancel my reminder about laundry at four o'clock."
Siri: I can delete reminders on your iPhone. (button saying Continue On iPhone)
There ya go. I think it's another instance of Apple's Human Interface Guidelines, where they say to, generally, make the user take an extra step before deleting something, hopefully preventing accidentally deleting data.