How can I disable alerts for a specific calendar on my Apple Watch

I tried what you suggested. I disabled event notifications for my Exchange account in the Calendar app on my iPhone. Then I set both "Notifications" and "Calendars" to "Mirror my iPhone" in the Watch app on my iPhone. I put both the watch and my phone on my desk, locked. At the next meeting reminder time, Outlook on my desktop reminded me of the event, but neither my phone nor my watch notified me.

Then, without changing any settings at all, I put the watch on my wrist and unlocked it and waited for the next meeting invite. This time, the watch notified me of the Exchange event, but my phone did not.

So the watch does not seem to be respecting the "Mirror my iPhone" setting when it is on-wrist and unlocked.

EDIT: For reference, this is a Series 6 Watch with watchOS 7.3. The phone is an iPhone 12 Pro with iOS 14.4.

I opened the Watch app on my phone, tapped Notifications, Calendar and for "Calendars", I selected "Custom" and unchecked the calendars for which I do not want notifications. Now, the events for that calendar do not appear in my calendar on the watch, but I am still receiving notifications for those events.

So far the only workaround I have found to this problem is to disable all alerts for calendar events on my watch, which is obviously not ideal.

I tried that (that is the default, I think) but I am continuously getting calendar reminders on my Watch that I am not getting on my phone. Worse, even though I "dismiss" an event reminder, the watch continues to remind me at 15, 10 and 5 minutes before the meeting.