How do I prune/clean wallet.dat for old unused addresses Are there any easy way to prune wallet.dat? Like, deleting keys for addresses with no current balance and generated more than a month ago?
Old addresses associated in the wallet will not cause harm.  Is there any specific reason you want to do this other than cleaning up the addresses associated in your wallet?  I would leave them where they are as you will normally always need new addresses for new transactions.

Or are you wanting to prune old addresses imported into a wallet.dat file that are now no longer in use?  In this instance I would be more tempted to find the addresses with funds in the on the wallet and import these into a new wallet.dat file and use this new wallet going forward.  

You can use Pywallet to prune old addresses out of the wallet but in the 14.0V of BTC i dont think there is a way.