This can be done in two ways without having to tinker with codes. First, it can be done with a custom field where users can input the URL and of which will be displayed on the ad. Second, you can enable HTML in the description area so that users can post a live link.
Can you elaborate further about "can I disable the email link" as I don't see any feature in the theme that displays the user emails in the ads.
So in simple terms, can each advert, in the detail of the product or service carry a link, without having to have a special bit of html or similar learning that a lay person would struggle with AND can I disable the email link.