Tell scraper to ignore a particular folder or folders.

I'm not aware of a way to do what you want. The easiest way is to move them out of your main Music folder into another folder and don't assign it as anything, that way it will not be scanned.

Unless you update your folder often, it doesn't really harm having them scanned as they will just error as not found.

Try this thing in the advancedsettings.xml file. The regex should work for files and folders.
PS: you need to create your own regex values...

<audio>
    <excludefromscan>
        <regexp>-trailer</regexp>
        <regexp>[!-._ \\/]sample[-._ \\/]</regexp>
    </excludefromscan>
</audio>

FYI: advancedsettings.xml - Official Kodi Wiki

Kodi has no limit on the number of sources you can have so you should expect to have more than one folder for music to manage different scraper requirements. In other words; the correct answer is in post #2.