Apache: "Header set Server" not working
That's because it's an Apache policy not to lie about the Server header and to always set it. The best you can do is to have it only display "Apache" with :
ServerSignature Off ServerTokens Prod
As it's said in the FAQ and a bug report, the only way you can have Apache send "My Server Name" is to modify the sources.
Note also that you won't gain anything by having your server not saying you're using Apache, security through obscurity is, well, not really security. If you're trying to hide your operating system or your php or ssl versions, in case a bug arise, it's not really useful either because people tend to try getting into every server they encounter.