Here is the story. I have suspended the website of a client of mine because of a lack of payment. Ok, this can happen. I have a reseller server, and I deleted his account and also all the DNS entries. The client did not change his nameservers, and the domain name is still pointing to my nameservers (let's say ns.domain.com). However, the domain is no longer responding and has no SOA entry. Now the funny thing is that google started to index my own website with the domain name of my ex-client and now my website shows up in the SERPs where used to show up my ex-client. This is completly crazy. Any advices would be really appreciated. Thanks in advance for your help!
This is probably a result of Google using cached DNS entries, and that you deleted the VirtualHost container for the non-paying client's Web site. Apache will default to the first VirtualHost in httpd.conf in the case where a requested hostname is undefined, and so your main site (defined as the default server) is being served.
This problem will cure itself once Google's DNS cache expires.
So, this is likely a pure server config problem, and has nothing to do with Google specifically, except that they cached your server's IP address in their DNS cache for the client's site.
You can work-around this problem by re-defining the client's domain on your server, and putting up (only) a custom 404 and/or 410-Gone page telling visitors (mostly robots) that the site is no longer there. Be careful what you say on these pages to avoid legal backlash.