Express Burn will not overwite an existing CD-R. A CD-R is a write one media. Express Burn is ejecting it as it is detecting an already burnt CD (it should say so).
You can append data to a CD-R when you write using multi-session writing. This allows to to add or even delete files from an already burned CD-R, but it is in-efficient as the old files al still present and you need about 12Mb to start a new session.
BUT
Express Burn does not currently support multi-session writing.
If you wish to completely erase and start again the only way is to use a CD rewritable (CD/RW) which can be erased and burn again