If you see any official announcement by BanDai on the TamaTown / Music City site that includes rules about not using them and a ban if they are used then it is likely that this is true.
But, if you have only "heard" about it from other members who can't even link you to the official announcement or rule about it then it is probably not true.
Lots of Music City users lie about if you are allowed to use the codes because they don't want others to use them too.
In some cases they make up stories about how they are an official "tester" for BanDai and that is how they know. The fact is, that unless BanDai have posted a rule that you will be banned for using the codes I don't see how it could be true...
One of the reasons that users don't want the codes to be shared is because a few members have spent a lot of time working out the codes and trying all the different combinations to get the special characters.
You can understand that they have worked hard to be able to use them and probably feel that it is not fair that others just ask for the code and use it when those others have not done any work for it.
There are also some users who got the code early on (without any hard work) and just don't want it shared so that they still have a "rare" code.
All I can add is that here on TamaTalk you are welcome to discuss the codes, but everyone is expected to follow our site rules and keep their posts friendly, polite and respectful.
If another TamaTalker is being rude or mean, don't think it is OK to break the rules too and post a rude reply. It's not OK. Just use the REPORT button (orange triangle with a !) that you can find in the bottom right hand corner of every reply box and one of the TamaTalk Guides will deal with it.