You can only ignore songs over a certain length. The purpose of this was so that random play wouldn't pick up that 75-minute version of the Hokey Pokey that you keep around for the kids...
If this is what you're looking for, check jukebox.ini for the MaxRandomSonglength setting. The default is 6 minutes. Note that this is not foolproof; if, for example, you set it to 1 minute and you have 1000 songs over 1 minute and only 5 under the limit, it will probably still pick longer songs.
--Chris