This should be simple enough. Some streamers have subs moved to VIP status and then set up song requests so that those who are VIP can request songs but those who aren't can't request any (ie, 0 songs).
If the maximum number of requests someone can make is '0', then instead of telling them that they have exceeded their limit, tell them that they cannot do song requests at all (or optionally ignore the request completely). I've seen where several users will try to request songs despite it being said that it's subs only and despite getting a message saying they've exceeded their limit. Either they get confused and believe that it will work if they keep trying or they may be attempting to flood the bot. If the message is changed or if it's set so that it won't respond (but only if they can't do requests at all), then it may cause less confusion. If set to just ignore them, it would cause less chat flooding from repeated requests, especially by some of the same people.