EMail choices/selections?

There are currently 4 eMail choices/selections which may be used when joining a Group (Individual eMail, Daily Digest, Special Notices, and NO eMail). Group's rules allow only Individual eMail and Daily Digest. Sometimes, prospective members cannot read or frequently ignore the rules hoping to be accepted as a member anyway. What can be done to make the Special Notices and No eMail options as inactive (disabled) options? It would seem a simple task to add a block which would enable/disable eMail options.

EMail choices/selections?
There are. If you own or are a Mod in groups and have the power to use it that is.