Let me pass
The last thing I want is someone to tell me I can't do something. Constantly telling me to stop when I want to go can be very annoying. Interrupting my work flow and shouting some message to me is rude.
Computers who stop humans doing what they are trying to do are rude. They are obtrusive and should not stick up big signs to tell the user to stop, but to unobtrusively act on errors and warnings. A computer should be a very smart polite person, they should work out what a user is trying to do and allow them to pass.
What we need to do is to be subtle but give enough information for the user to pause for a moment and change their input. We should do this diligently and quietly. Your users will end up being more happy and have less chance of leaving your website.