Take a look at the file path for some of the images and videos that are in your library that you don't want there. I'm guessing they are in a subdirectory underneath one of your auto import folders? As far as I know, MC only imports from the folders you configure in auto import. ...and of course it imports whatever you explicitly tell it to, on a directory basis.
If your photos and videos are in directories below one of your auto import folders, never fear. You can tell MC to only import certain types in certain directory structures.
Tools > Options > Library & Folders > Configure auto import
Look to see what directories you have set up for auto import and if they match the file path of some of your rogue photos or videos. If so, click on the folder of interest and click Edit. Then under "Import These File Types", uncheck the types you don't want to have imported.
For the existing photos and videos that you don't want to see (that have already been imported), you can either roll back to an earlier library backup, or just find all of the photos and videos (with an expression or a smartlist) and delete them from MC's library. By default, when you delete from the library, MC does NOT touch the files. Be sure to take the default option of "only remove from library" and you'll be safe.
Let us know if this helps, or if you need help in a different direction.
Brian.