Is it safe to keep a compromised private key in my wallet? I'm using bitcoin-qt.  Suppose that one of the private keys in my wallet is compromised somehow, and is released to an attacker or the public.  Of course, I can transfer all the coins from the compromised address to a new one (unless the attacker does it first).  However, bitcoin-qt apparently doesn't provide a way to delete or disable a private key from a wallet.   (This is understandable; it would be pretty bad to accidentally delete a private key for an address that had coins in it.)

Is it safe to leave the key in the wallet, assuming I never again intentionally transfer any coins to that address?  (I could label it DONOTUSE or something.)  Or are there situations (e.g. change) where the client might automatically transfer coins there without asking me?
If it is only one private key and you are aware not to use it anymore (and have rerouted any anticipated transactions to a new address), there is nothing harmful that the attacker can do.

Bitcoin-qt sends change from transactions to new unused addresses. Since those are the only transaction outputs that are transmitted to not-explicitly-specified addresses, it is safe to keep the private key in your wallet.

A compromising party could, however, use the private key to sign messages. These messages would appear to be authored by you to recipients that know the respective address to be under your control. Unless the compromising party had additional information about you and the counter-parties of previous transactions the compromised key was used in, this shouldn't be useful to them (nor could you stop it either).