Yes they can....it's not very hard. Some mild cases only need encouragement ie training to teach them people = Fun toys and treat machines. Some harsher cases can't be FIXED per say, but controlled, through strict obedience. If you have a dog who's fear aggressive, yet obeys your commands 100% of the time, he'll never get a chance to bite somebody. This isn't a cure, it's a cover up but you do what you can, with what you've got.
As I always say Genetics = What you're given to work with
Training = What you do with what you've got
I can have a weak temperamented slightly fearful dog who's a much better dog than your genetically perfect dog, if you don't invest the time to train the dog. You can not FIX genetic fear, you can TRAIN a genetically fearful dog that the things it fears aren't scary things through positive methods, and you can advert negative outcomes with corrections training. But no, it can not be fixed....but to me rehabbing isn't fixing, it's dealing. So yes, fearful dogs can be dealt with, they can be rehabbed, some can be "cured"...it depends on the trainer, the dog, the owner, and the amount of time you're willing to dedicate to it.
-Owner of two fearful rescues, and 1 that was a full blown fear aggressive, obnoxious ankle biter who gets on through daily life just fine now.
-edit- Okay I see some talk that a fearful dog my bite somebody out of some random trigger....this is ABSOLUTELY TRUE...but so can a VERY WELL TRAINED stable dog. I can tell you what my dogs will PROBABLY do, but no man on this planet can say a dog won't bite a guest or visitor because a dog's brain weighs 3 fricken ounces. Very funny in fact because...Michael Ellis spoke to that point just a few weeks ago...about not being about to say any dog won't bite.
Michael (rough quote): "I mean for protection dog trainers to advertise that their dogs are completely safe around strangers is BULL, and a lot of sport people will say "Ohhhh, sport trained dogs don't bite because we train them when not to bite!"...they're trying to protect their sport, understandable, but they're lying...period...the dogs may bite somebody indeed...but so may the average house pet...and I'll even go as far as to say the average house pet is MORE likely to have a misunderstanding and bite a guest or stranger...but to say that accidents don't happen when dealing with animals, trained or untrained, and of good and bad temperaments...is simply untrue."
I agree with that statement 100%...
When people ask me: "Does your dog bite" I simply say: He doesn't bite me. I never allow strangers besides the vet, to touch my dog. In the dog sporting game you have helpers who pet dogs during work...I don't agree nor disagree with that, and I DO believe the helper should and can correct/praise the dog, I simply ask him to leave the physical praise to ME.