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

0. Make a list of all the private keys you do care about.
1. Close your client.
2. Move the wallet.dat out of the data directory to some other location.
3. Start the client.
4. 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.

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

0. Make a list of all the private keys you do care about.
1. Close your client.
2. Move the wallet.dat out of the data directory to some other location.
3. Start the client.
4. 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.