Or maybe you can add a filter? If you say Ya'll suck go **** the filter maybe says: You are not allowed to choose this message!A way to kinda add this could be having predetermined messages. So when you activate the multiplier, you can select a message out of a list ranging from ‘Hello all!’ to some silly messages, or whatever.
Then with every purchase of a multiplier, you could submit a message that might be added to the list directly allowing the community to contribute to the list. Of course any swear words or inappropriate messages would not be added. :D