Simply deleting them is fine, just make sure you exit the client first.
Last time I did that (on my empty wallet) the bitcoin client kept crashing when I relaunched it (ran for a few minutes then crash), and I eventually had to wipe out the whole blockchain and start over. Are you really sure I can delete all those files, or do you mean the old ones that haven't been modified for at least a week?
Someone else would have to confirm the technicalities, but I've never had any crash problems deleting log files, or any files other than the blockchain+wallet in fact.