blocking access to addentry.php

ok, another question that I'm sure has been addressed, I just can't find the answer.

Is there any way to block direct access to the addentry.php page without first going through the index.php?

Not simply no. You would have to log all ips that request index.php and when they requested it. Then check them that when they request addentry.php. No other method is feasible. Not everybody accepts cookies and not everyone sends a referer.