I haven't the energy to look it up, but in the manual it does explain something about using 'database.ignore' files. Basically put file with the appropriate name inside any directory you _don't_ want to be included in the database. If I remember correctly. Something like that, anyway!
Here it is (always read the manual first!)
Ignoring Directories During Database Initialization
You may have directories on your player whose contents should not be added to the database. Placing a file named database.ignore in a directory will exclude the files in that directory and all its subdirectories from scanning their tags and adding them to the database. This will speed up the database initialization.
If a subdirectory of an ‘ignored’ directory should still be scanned, place a file named database.unignore in it. The files in that directory and its subdirectories will be scanned and added to the database.