how to prevent a certain value in a field

Applications Forms
#1

How can I prevent a user from entering a certain value in a field?

In Onchange event I can check the new value, and give an error. But the new value is already in the field and can be saved…

Any other suggestions?

#2

Re: how to prevent a certain value in a field

Can you not clear the value and set focus?

You could also check onvalidate to no allow save of this value?

Regards
Scott.