This isn’t possible because observations are never really “included” in a collection project. When a collection project page is loaded, iNaturalist searches for observations that meet its requirements and displays those observations. It’s basically a saved set of search filters.
Likewise, when an observation page is loaded, something similar happens - iNaturalist searches for observations joined by the observer, then searches those projects that the observation fits into and displays those projects.
There’s this feature request (looks like you’ve commented on it but haven’t actually voted for it at the top of the page): https://forum.inaturalist.org/t/restrict-your-observations-from-appearing-in-someone-elses-collection-project/18502