Can I 'un-import' private key?
There is no way to 'un-import' a private key in the core client. What you could do instead is:


Make a list of all the private keys you do care about.
Close your client.
Move the wallet.dat out of the data directory to some other location.
Start the client.
Re-import all the private keys that you do want in your wallet.


This is sort of a work around, and it may be kind of tedious, sorry there isn't a better solution currently.