The only option you have is to make events not visible to certain users, e.g. anonymous users. But then they wouldn't show up in the proper calendar (for those users) either.
If you have a reliable criterion to tell these events apart from others, you could make a copy of function phpblock_calendar (from the Calendar plugin's functions.inc) and adapt it to your needs (create a
PHP block for your new function).
bye, Dirk