Is it possible to prevent users from changing standard fields?

Here's what I want to do:

My custom appointment form starts up an application that writes to a SQL
database. When I've stored the record I can track and either undo them or
get my app to replicate the changes dynamically. This is proving rather
difficult to manage and it would be much easier if I could just stop them
doing it after a critical point has been reached. For instance, changing
start time will require constant monitorting and the corresponding event
seems to be fired off repeatedly when this attempted.

I realise I can save my item and close it but I need it to remain open to
allow changes to fields that don't affect my database content. Is there a
way of doing this that anyone knows of?

Jeff Mowatt