Spoetryinmotion, sorry to hear that your horse got that way. I have sen incredibly treat spoiled horses being trained (as strange as it sounds) to be 'treat-safe' again by clicker training, using experienced trainers who had great timing. I have never had nearly as bad a case, my mare got a little treat grabby in the beginning (my own fault as I was in the beginning stages of learning) but since then, and by clicker training, she is now incredibly polite about treats and knows to 'look away' before she will get a treat.
Marybonus, clicker training for horses is really a fantastic aspect of training that combines wonderfully with other types of training. To get started, I would do some research, both on the internet and in books if you have access to them. Karen Pryor and Alexandra Kurland are great clicker trainers and I know there are many more. Then I would start with the four (five) basic lessons. The first, charging the clicker may or may not work, depending on the horse (my mare only made the association when I started the next lesson), then there's target training, happy face/politeness, matwork and backing. Whilst I'm no expert at all, I'd be happy to try answer any questions you might have.
Ledge, I'm curious as to why you would think that clicker training could be unsafe? And, just to point out, there are also different levels of treat rewards. My mare works much harder for pony nuts then apples or carrots so I reserve them for teaching a new behaviour.