Apache shows the first vhost in alphabetical order for a IP address if there is no vhsot for a given domain. So what you describe is the normal behaviour of the apache webserver when it does not find a site because you disabled it.
To redirect the requests to a specific site, you have to create a default vhost that catches these requests.
Create a new website with this domain name:
and select the IP address that it shall work for (the Ip that the other sites use). Now all requests of disabled sites will go to this vhost instead of a vhost of a different customer.