You can't copyright the alphabet.
You can only copyright the specific code that makes up your font, for which you need to come up with usage licensing.
You do need to register it before you ever need to go to court for infringement.
You can tell them that someone always manages to get away with theft (yes, start calling it theft), but that doesn't make it okay. Remind them that if they choose to steal, they risk a cease & desist,...