I usually create a new Library for a new project, to do so, hold Option and open the Photos app, which brings a menu from where you can create a new Library, or open an existing Library, other than the one currently opened.
That's fine if not cumbersome. Sometimes though I import pictures into the wrong Library...
To fix this, I Export the pictures in question and put them into a temp folder. When that's done, I delete the pictures - but you will notice that the files are still present in the wrong Library at the disk / Finder level... No shrinkage in size for that Library at the Finder level.
To delete the pictures for good from the Library/drive, select in Photos: File/Show Recently Deleted, and then click on the Delete All button on the top left corner. Photos will warn you that the pictures will be gone forever... Yes! Exactly what I want.
After some time... Variable depending on the amount of pictures you are throwing away I suppose, the Library file will shrink, indicating that indeed the photos have been deleted from the drive and from the "wrong" Library.
Last part, open the correct Library: quit, launch Photos again while holding Option + select the correct Library from the menu. Then Import the pictures from the temp folder. Done
