This is an age old problem with no easy answer. It is unusual for your dog to be so persistent, for the following reasons in particular: a) He is being told-off very severely every time he tries; b) He is old and may not have the stamina or desire as younger dogs; c) if your female is neutered she should not be so attractive to males. The first thing I would do is make sure that your dog really is neutered. If she really is, then I can suggest nothing other than to separate the dogs when you are not present to stop him from ‘misbehaving’!