If the site is the primary domain, you cannot close it yet. If you want your current site unreachable immediately, put it in maintenance mode. Register a new domain related to your new niche and point DNS to your host provider. Get in touch with your host provider, request to make the new domain your primary domain. Install wordpress, theme and everything to the new domain. When your current domain is not the primary, you can request it to be in a parked domain or have it deleted.