I need some help on validation.

User input value X is restricted by other user input values found in
other cells. There are so many of these, with so many rules, I've
decided to do it in code.

There must be a begin calculations event. I'll jump in there and
validate. If bad data is found I will notify the user and stop wb
processing. But how to stop? If calculations can be aborted, how do
I turn it "on" again? The user still needs to enter data and I still
need to check it.

Just looking for ideas before I try and implement something and have
to do it all over again later.

Thank you.