Prevent checkout

[VeraVault]

Hi,

 

I wrote a hook that checks a custom field for certain criteria. (checking if an email exist)

I'm using the "PreCalculateCartTotals" hook. How can I stop a client from checking out at this point?

 

for example, I'd like this hook to work just like the "Regular Expression Validation" to prevent a certain value from being used.

 

Ideas?

 

Thanks in advance

[VeraVault]

cant be done?