Depends what type of files you are trying to download, but gnutella is not the best for program sharing. Normally the largest files in size are the best. Try to stay away from files below 2,000 KB (2 MB).

You can right-click the file in LW & do a bitzi search to see if it's reported.
If the files are spammed junk then yes you can flag them for yourself as being junk but clicking the trash can icon after selecting the file.