I would not follow the advice to smack him in the mouth. This tends to just cause them to want to bite more, as now you have engaged in the fight. I would go into an open area with a long rope. Start grooming, taking off the blanket, etc. basically whatever you would normally do. When he tries to bite, immediately send him away and make him WORK. Circles, changing direction, backing up, etc. When he appears more submissive (head down, chewing, coming closer to you) then pick up where you left off. Do not make a fuss about it. This may take quite a few times for a really stubborn horse, and you will have to reinforce this every time you work with him in any way. Remember, he has been doing this for 7 YEARS, you are not going to get it to stop in a few minutes. Doing more groundwork with this horse may also be required. Get him to respect your space and follow directions...do not get mad, this is counterproductive. If you meet force with force, an understanding is impossible. You need to get through to this horse's thought pattern to modify such an ingrained behavior. Obviously, if this is not your horse, this may not be possible. If the trainer is unwilling to do anything about it, you may have to resign yourself to the fact that this is going to be an ongoing problem and you just need to take steps to protect yourself. Unfortunately, many "trainers" aren't willing to put in the time to actually correct a problem and just use cover-ups to mask them. I have known many people like this and ultimately I have found that I am happiest when I find a situation where I am around people who use more intelligent methods, even if that means switching instructors or barns.