Field type validator:
When you assign validations to a field, the changes you have made to the field affect all the forms on your website that use it. In addition to the default validations, you can also create your own custom field-type validations.
What you did, is the validation for the Field Type not for a specific form. You can create predefined validations which is documented here - https://doc.sitecore.net/web_forms_for_marketers/working_with_actions_and_validations/validations/set_up_and_create_a_formspecific_field_validation
Unfortunately this only allows you to put there regular expressions.
I would create form verification which is called when the user submit the form.
After you created the new Form Verification item you can select this on the form.
The implementation should look like something like this:
public class KeyValidator : WffmCheckAction
public override void Execute(ID formid, IEnumerable<ControlResult> fields, ActionCallContext actionCallContext = null)
// here is your validation