Technically, ActiveDEMAND cannot block people from your website. ActiveDEMAND does have some great features to help you control access to content though.
The 'block account' feature in ActiveDEMAND is a great way to block users from accessing your material. Click any account and check the 'Block Account' checkbox, add an associated email domain to the account, and you now have blocked the competitor.
Essentially, an employee of a blocked account will never get an auto-responder from a webform on your site. As well you can re-direct the people to a separate 'confirmation' url that says something nice like "Thank you for your request, someone will get back to you shortly". It is important to note that typically an account will have an email domain associated with the account. Thus any form submit that has a contact with an email that matches the account domain will be added to the Account. Hence anyone who submits a form with an email address that is from a domain that is associated with a blocked account will be blocked.
What if a contact fills out a company field on a form, and the company field differs from the blocked account in ActiveDEMAND?
Say you have an account: Acme Inc.. Acme is a competitor of yours. You have the email domain acme.com is associated with the account. Hence any person who submits a form with an email address @acme.com will be 'blocked'. If you have a 'Company' field on a form, and someone puts in Acme Wholesale as the company name and uses an @acme.com email address with the form. ActiveDEMAND will create a new account "Acme Wholesale" and the email address will indicate that this new account must be blocked as well. Thus the contact will not get the resource. It is important to know that acme.com is still associated with Acme Inc, not Acme Wholesale.
You could as well use the spam functionality to block a form submit. You could label a domain as a spam domain, and all form submits will be quarantined and the person will not get the autoresponder nor the resource.