What exactly is preventing the update?  Is it because the input is of the
incorrect type or is it that the data is invalid?  Does the message result
from validation you are performing?  You should be able to do a Cancel = True
in your BeforeUpdate event, but this will not revert the input.  Consider the
case if it did - the user would enter in bad data that he thought was good
and the program would automatically reset it.  Unless he knew that the
program was supposed to do this, he might suspect that the application was
broken.  In cases where the user is trying to enter bad data that he thinks
is good, he will definitely think that the application is broken.  I suggest
that you add an alert message that tells the user the data is bad and to
force him to fix it, perhaps via the Escape key.  If your problem is a result
of a validation that you do, you could allow the update and then reset the
value using the property OldValue.