how can i prevent crown from activating siri?

You can't as of right now... :/ Sorry to have such a disappointing solution.

You can, however, suggest that Apple include this feature in a future WatchOS update: Feedback - Apple Watch - Apple

If the Digital Crown is continually activating, you could also switch the orientation of the Digital Crown on your wrist by going on your Apple Watch to Settings>General>Orientation and choosing a different option under 'Digital Crown.'